One-step synthesis of hydrophobic fluorinated ordered mesoporous carbon materials

Abstract

A series of fluorinated ordered mesoporous carbon (FOMC) materials were prepared by a one-pot method. The FOMC materials were characterized by X-ray diffraction, N₂ adsorption–desorption isotherms, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The FOMC materials have well-ordered mesoporous structures with large specific surface areas and uniform pore sizes. Contact angle analysis showed that the FOMCs were more hydrophobic than the corresponding unfluorinated parent material. Thus the incorporation of fluorine improved the hydrophobicity of the carbon materials. This method provides a fast and easy-to-scale-up method for producing ordered mesoporous carbon materials with high hydrophobicity.

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1. Introduction

Ordered mesoporous carbon materials (OMCs) have attracted great interest due to their well-defined mesoporous structures, their large surface areas and their large pore volumes. Typically OMCs are obtained using mesoporous silica in nanocasting or self-assembly template methods. As an important modification, fluorination can improve the properties of graphite, activated carbons, and carbon nanotubes. The fluorinated carbon materials are widely applied in electrical conductors, primary and secondary batteries, electromagnetic materials and aerospace technology. Recently the surface functionalization and modification of mesoporous carbon materials has been widely used in the field of OMCs for adsorption, separation and catalysis applications, which require adsorption materials or a catalyst carrier with a large aperture and high specific surface area. The incorporation of fluorine into OMCs has been reported to improve the hydrophobic properties of the materials.^1,2

Wang et al. used fluoroalkylsilane as a fluorine precursor to modify carbon monoliths and obtained ultra-hydrophobic mesoporous carbon monoliths.³ Fluorinated mesoporous carbon has been obtained by the direct surface modification of CMK-3 using fluorine gas.^4,5 However, it is difficult to control the fluorine content in post-synthetic fluorination methods and the harsh reaction conditions can induce the collapse of the pore structures and generate surface defects. Therefore, template methods which do not damage the carbon mesostructure and which can directly introduce heteroatoms into a carbon precursor are more suitable for the fluorination of OMCs. Wan et al.⁶ prepared fluorinated ordered mesoporous carbons by using an evaporation induced self-assembly (EISA) method.

The current focus on reducing organic solvents in industry has prompted the development of aqueous phase OMC syntheses.^7–9 Compared with EISA methods, aqueous methods reduce the consumption of organic solvents, and have lower production costs. Moreover, an additional pre-polymerization step is not necessary in aqueous phase synthesis, and since organic solvents are not used during the self-assembly, high purity OMCs with no heteroatoms are obtained. Herein a novel preparation method for fluorinated ordered mesoporous carbons (FOMCs) with different fluorine contents via a one-pot aqueous self-assembly strategy is reported.

2. Experimental

2.1 Materials and synthesis method

Triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymer Pluronic F127 (MW = 12 600, PEO₁₀₆PPO₇₀PEO₁₀₆) and p-fluorophenol were purchased from Aldrich Corporation, USA. The other reagents were purchased from Guangfu Chemical Corporation, China. All chemicals were analytical grade and were used as received without further purification.

A simple schematic representation of the FOMC synthesis process is shown in Scheme 1. First hexamethylenetetramine (HMT) was dissolved in water and then hydrolyzed to form formaldehyde and ammonia by controlling the temperature. The formaldehyde then served as a reactant in the subsequent polymerization reaction with p-fluorophenol and resorcinol. The ammonia remained dissolved in the aqueous reaction solution to give alkaline reaction conditions which provides a stable alkaline catalysis for the polymerization reaction. Finally self-assembly with F127 was allowed to take place resulting in a fluorine containing mesoporous material.

Scheme 1 The synthetic route for the FOMCs.

The ratios of the reaction components p-fluorophenol/resorcinol/HMT/F127/water (molar ratio) were x : (1 − x) : 2 : 0.05 : 5, where x denotes the molar fraction of p-fluorophenol compared to resorcinol and p-fluorophenol. In a typical procedure, 0.55 g of resorcinol (x = 0.5), 0.56 g p-fluorophenol, 0.70 g of HMT, 2.20 g of Pluronic F127, 52 mL of deionized water and 2 mL of aqueous ammonia (28 wt%) were randomly mixed. The mixture was then stirred under reflux at 80 °C for 24 h. The product was isolated by filtration and then calcined at 350 °C for 5 h under nitrogen to remove the template. The product was then further treated at 500 °C under nitrogen for 5 h and the resulting composite was labeled as FOMC-x.

2.2 Catalyst characterization

X-ray diffraction (XRD) analysis was carried out using a Rigaku D/Max 2500 X-ray powder diffractometer. N₂ adsorption–desorption isotherms were measured at 77 K using a Micromeritics Tristar 3000. The morphologies of all the samples were characterized by high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G²F20). Fourier transform infrared (FTIR) spectroscopy was recorded on a Nicolet 6700 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI-1600 spectrometer equipped with Mg Kα radiation. Morphologies of the water droplets were obtained using a contact angle analyzer (OCA15EC, Dataphysics, Germany). For each measurement, three random locations on each sample were measured and the average values are reported.

3. Results and discussion

The low angle XRD patterns of the FOMCs with different amounts of fluorine are shown in Fig. 1A. FOMC-0.2, FOMC-0.3, FOMC-0.4 and FOMC-0.5 all have an intense diffraction peak centered at around 2θ = 0.9°. This peak is due to the (100) diffraction of a hexagonal mesostructure,¹⁰ and indicates that these four samples have well-ordered hexagonal structures. In contrast, the other three samples (x = 0.6, 0.7 and 0.8) do not have any obvious diffraction peaks suggesting that these materials have disordered structures. Therefore, a fluorine content in the range of 0.2–0.5 is favorable for the formation of a FOMC with a well-ordered mesoporous structure.

Fig. 1 Low-angle (A) and wide-angle (B) XRD patterns of the FOMCs.

The wide angle XRD patterns of these four samples are shown in Fig. 1B. There are two broad peaks centered at 23° and 43°, which are due to the (002) plane of disordered amorphous carbon and the (100) plane of graphite-like carbon,¹¹ respectively. These results indicate that the FOMC frameworks are composed of amorphous carbon.

The N₂ adsorption–desorption isotherms and pore size distribution curves for the FOMCs are shown in Fig. 2, and the corresponding textural properties are summarized in Table 1. All of FOMCs samples exhibit typical type IV isotherms with a H1-type hysteresis loop (Fig. 2A), indicating that these materials have mesoporous structures. The pore size distributions of the FOMCs samples are very narrow and centered at about 3.3 nm for all samples (Fig. 2B), which were larger than the materials reported by Cao,¹² as the fluorine content increased, the specific surface area and pore volume of the materials decreased. This can be attributed to the incorporation of fluorine into the surfaces of the materials.

Fig. 2 N₂ adsorption–desorption isotherms (A) and pore size distributions (B) of the FOMCs.

Table 1 Textural properties and fluorine content of the FOMCs

Sample	Specific surface area (m^2 g^−1)	Pore volume (m^3 g^−1)	Average pore size (nm)	F content (wt%)
				EDX	XPS
FOMC-0.2	622	0.40	3.3	1.72	1.62
FOMC-0.3	613	0.39	3.2	2.65	2.53
FOMC-0.4	570	0.34	3.4	3.53	3.34
FOMC-0.5	559	0.32	3.3	4.45	4.36

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The TEM images of the FOMC samples (Fig. 3) show that the samples have well-ordered mesoporous structures. As the fluorine content increased, there were no changes in the FOMC pore sizes, which is consistent with the BET pore size results.

Fig. 3 TEM images of the FOMCs.

The FTIR spectra of the FOMCs are shown in Fig. 4. The bands at 1246 cm⁻¹ can be attributed to the C–O groups in the phenolic aldehyde resins. The peaks at 1610 cm⁻¹ are due to the aromatic-like C=C stretching modes in the polyaromatics and the peaks at 1420 cm⁻¹ correspond to the C–H bending modes of the CH₂ methylene units.¹³ The peaks centered at about 1080 cm⁻¹ can be assigned to C–F stretching vibrations,⁶ indicating that fluorine atoms are attached to the surface of the materials via C–F covalent bonds. The intensity of this peak gradually increases as the FOMC fluorine content increases.

Fig. 4 FT-IR spectra of the FOMCs.

The XPS fluorine spectra of the FOMCs are shown in Fig. 5. The peaks with binding energies of approximately 686.9 eV are due to carbon–fluorine groups.^14,15 This again indicates the successful incorporation of fluorine into the materials.

Fig. 5 XPS spectra of the FOMCs.

The amount of fluorine in the FOMCs was determined by EDX (energy-dispersive X-ray spectroscopy) and XPS, and the results are shown in Table 1. The results are in good agreement with each other. When the molar ratio of p-fluorophenol in the initial mixture was 0.2, the fluorine content after the carbonization treatment was about 1.72 wt% and a p-fluorophenol molar ratio of 0.5 gave an FOMC with 4.45 wt% fluorine, indicating that the measuring values matched well with the theoretical calculating values and there was almost no fluorine loss in the carbonization process. The FOMC with different fluorine content can be obtained by controlling the ratios of the reaction components. The one-pot aqueous self-assembly preparation method could regulate the fluorine content and avoid structural degradation if modified by fluoride modifier.⁴

The hydrophobicities of the materials were determined by measuring the contact angles of water droplets on the surfaces of the materials. A larger contact angle indicates a more hydrophobic material. The contact angles of water on the surfaces of the parent OMC and the OMCs are shown in Fig. 6. The FOMCs have higher water contact angles than the unfluorinated OMC sample, indicating that the incorporation of fluorine improved the hydrophobicity of the mesoporous carbon materials, and the prepared material have a better hydrophobic performance than the F-MCFs which reported by Cao.¹² In addition, the hydrophobicity of the FOMCs increased with increasing fluorine content, which means the hydrophobic properties of FOMC could be regulated by fluorine content in one-pot aqueous self-assembly process. The FOMC-0.5 with a water contact angle of 147.7°, which is closed to the super-hydrophobic materials (contact angle > 153°).¹⁶

Fig. 6 Contact angles of water droplets on OMC and FOMCs.

4. Conclusions

A series of fluorinated ordered mesoporous carbon materials was synthesized via a one-pot aqueous self-assembly process using p-fluorophenol as the fluorine source. The FOMCs are more hydrophobic than the unfluorinated parent OMC, and the hydrophobicity of the FOMCs increased with increasing fluorine content. A hydrophobic carbon materials with high specific surface areas, uniform pore sizes and well-ordered mesoporous structures comes out and exhibits its industrial applications rely on its cheap and convenient new routine to prepare.

Acknowledgements

The authors are grateful for the financial support from the National Scientific Foundation of China (No. 21104035) and the SINOPEC (No. 415088).

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