One-step synthesis of hydrophobic fluorinated ordered mesoporous carbon materials

Article abstract of DOI:10.1039/c6ra06583j

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.

Full text of DOI:10.1039/c6ra06583j

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One-step synthesis of hydrophobic fluorinated ordered mesoporous
carbon materials
Xiuqin Dong, Xueshuang Zhao, Lingtao Wang, Minhua Zhang*
Key Laboratory for Green Chemical Technology of Ministry of Education, Research and Development
Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China
*Corresponding authors: Tel./fax.: +86 22 27406119
Eꢀmail address: mhzhang@tju.edu.cn (M. Zhang)
Abstract
A series of fluorinated ordered mesoporous carbon (FOMC) materials were
prepared by a oneꢀpot method. The FOMC materials were characterized by Xꢀray
2
diffraction, N adsorptionꢀdesorption isotherms, highꢀresolution transmission electron
microscopy, Fourier transform infrared spectroscopy and Xꢀray photoelectron
spectroscopy. The FOMC materials have wellꢀordered 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 easyꢀtoꢀscaleꢀup method for producing ordered
mesoporous carbon materials with high hydrophobicity.
Keywords: Carbon materials; Fluorinated ordered mesoporous carbon; Hydrophobicity;
Softꢀtemplate method; Mesoporous materials.
1. Introduction
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Ordered mesoporous carbon materials (OMCs) have attracted great interest due to
their wellꢀdefined mesoporous structures, their large surface areas and their large pore
volumes. Typically OMCs are obtained using mesoporous silica in nanocasting or
selfꢀassembly 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 electric conductors, primary and
secondary battery, 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, separations and catalysis applications，
which require adsorption materials or catalyst carrier with large aperture and high
specific surface area. The incorporation of fluorine into OMCs has been reported to
1, 2
improve the hydrophobic properties of the materials.
Wang et al. used fluoroalkylsilane as a fluorine precursor to modify carbon
3
monoliths and obtained ultraꢀhydrophobic mesoporous carbon monoliths. Fluorinated
mesoporous carbon has been obtained by the direct surface modification of CMKꢀ3
4
, 5
using fluorine gas.
However, it is difficult to control the fluorine content in
postꢀsynthetic 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
6
OMCs. Wan et al. prepared fluorinated ordered mesoporous carbons by using an
2

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evaporation induced selfꢀassembly (EISA) method.
The current focus on reducing organic solvents in industry has prompted the
7
ꢀ9
development of aqueous phase OMC syntheses. Compared with EISA methods,
aqueous methods reduce the consumption of organic solvents, and have lower
production costs. Moreover, an additional preꢀpolymerization step is not necessary in
aqueous phase synthesis, and since organic solvents are not used during the
selfꢀassembly, 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 oneꢀpot aqueous selfꢀassembly strategy is reported.
2. Experimental
2.1 Materials and synthesis method
Triblock poly (ethylene oxide)ꢀbꢀpoly (propylene oxide)ꢀbꢀpoly (ethylene oxide)
copolymer Pluronic F127 (MW = 12600, PEO106PPO PEO106) and pꢀfluorophenol
70
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
pꢀfluorophenol and resorcinol. The ammonia remained dissolved in the aqueous
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reaction solution to give alkaline reaction conditions which provides a stable alkaline
catalysis for the polymerization reaction. Finally selfꢀassembly with F127 was allowed
to take place resulting in a fluorine containing mesoporous material.
N
△
N
+
6H
2
O
6HCHO
+
4NH3
N
N
NH3
+
H
O
2
NH
4
+
OH
OH
OH
H
C
2
F
F
OH
HO
OH
OH
OH
+
OH △
+
HCHO
H2C
2
H C
OH
OH
F
CH
2
F
F
Scheme 1. The synthetic route for the FOMCs.
The ratios of the reaction components pꢀfluorophenol/resorcinol/HMT/F127/water
molar ratio) wee x:(1ꢀx):2:0.05:5, where x denotes the molar fraction of pꢀfluorophenol
(
compared to resorcinol and pꢀfluorophenol. In a typical procedure, 0.55 g of resorcinol
x = 0.5), 0.56 g pꢀfluorophenol, 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 ºC for 24 h. The product was isolated by
filtration and then calcined at 350 ºC for 5 h under nitrogen to remove the template. The
product was then further treated at 500 ºC under nitrogen for 5 h and the resulting
composite was labeled as FOMCꢀx.
2.2 Catalyst characterization
Xꢀray diffraction (XRD) analysis was carried out using a Rigaku D/Max 2500
4

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2
Xꢀray powder diffractometer. N adsorptionꢀdesorption isotherms were measured at 77K
using a Micromeritics Tristar 3000. The morphologies of all the samples were
characterized by highꢀresolution transmission electron microscopy (HRTEM, FEI
2
Tecnai G F20). Fourier transform infrared (FTIR) spectroscopy was recorded on a
Nicolet 6700 spectrophotometer. Xꢀray photoelectron spectroscopy (XPS) was
performed on a PerkinꢀElmer PHIꢀ1600 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. FOMCꢀ0.2, FOMCꢀ0.3, FOMCꢀ0.4 and FOMCꢀ0.5 all have an
intense diffraction peak centered at around 2θ = 0.9°. This peak is due to the (100)
10
diffraction of a hexagonal mesostructure, and indicates that these four samples have
wellꢀordered 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 0.2ꢀ0.5 is favorable
for the formation of a FOMC with a wellꢀordered mesoporous structure.
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
11
disordered amorphous carbon and the (100) plane of graphiteꢀlike carbon, respectively.
5

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These results indicate that the FOMC frameworks are composed of amorphous carbon.
(A)
FOMCꢀ0.8
FOMCꢀ0.7
(002)
(B)
(
100)
FOMCꢀ0.5
FOMCꢀ0.4
FOMCꢀ0.6
FOMCꢀ0.5
FOMCꢀ0.4
FOMCꢀ0.3
FOMCꢀ0.2
FOMCꢀ0.3
FOMCꢀ0.2
FOMCꢀ0.1
1
2
3
4
5
10 20
30
40 50
60 70
80
2
θ
(degree)
2
θ
(degree)
Fig. 1. Lowꢀangle (A) and wideꢀangle (B) XRD patterns of the FOMCs.
280
260
240
220
200
180
160
140
120
(A)
0
.020
.015
(B)
0
FOMCꢀ0.2
FOMCꢀ0.3
FOMCꢀ0.4
FOMCꢀ0.5
0.010
FOMCꢀ0.2
FOMCꢀ0.3
FOMCꢀ0.4
FOMCꢀ0.5
0
.005
.000
0
0
.0
0.2
0.4
0.6
0.8
1.0
2
4
6
8
10
Pore size (nm)
Relative pressure (P/P )
0
2
Fig. 2. N adsorptionꢀdesorption isotherms (A) and pore size distributions (B) of the FOMCs.
2
The N adsorptionꢀdesorption 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 H1ꢀtype
hysteresis loop (Fig. 2. A), indicating that these materials have mesoporous structures.
The pore size distributions of the FOMCs samples are very narrow and centered at
6

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about 3.3 nm for all samples (Fig. 2. B), which were larger than the materials reported
12
by cao , 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.
Table 1. Textural properties and fluorine content of the FOMCs.
Sample
Specific surface
Pore volume
Average pore size
F content (wt %)
2
3
area (m /g)
(m /g)
(nm)
3.3
EDX
1.72
2.65
3.53
4.45
XPS
1.62
2.53
3.34
4.36
FOMCꢀ0.2
FOMCꢀ0.3
FOMCꢀ0.4
FOMCꢀ0.5
622
613
570
559
0.40
0.39
0.34
0.32
3.2
3.4
3.3
Fig. 3. TEM images of the FOMCs.
FOMCꢀ0.5
FOMCꢀ0.4
FOMCꢀ0.3
FOMCꢀ0.2
-
1
1
1
080cm
-
1
-
1
610cm
2000
-1
1
246cm
1
420cm
1500
4000
3500
3000
2500
1000
500
ꢀ
1
Wavenumbers
(
cm
)
Fig. 4. FTꢀIR spectra of the FOMCs.
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FOMCꢀ0.5
FOMCꢀ0.4
FOMCꢀ0.3
FOMCꢀ0.2
680 682 684 686 688 690 692 694
Binding Energy (eV)
Fig. 5. XPS spectra of the FOMCs.
The TEM images of the FOMC samples (Fig. 3) show that the samples have
wellꢀordered 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.
ꢀ1
The FTIR spectra of the FOMCs are shown in Fig. 4. The bands at 1246 cm can
ꢀ1
be attributed to the CꢀO groups in the phenolic aldehyde resins. The peaks at 1610 cm
are due to the aromaticꢀlike C=C stretching modes in the polyaromatics and the peaks at
ꢀ1
13
1
420 cm correspond to the C–H bending modes of the CH
2
methylene units. The
ꢀ1
6
peaks centered at about 1080 cm can be assigned to CꢀF stretching vibrations,
indicating that fluorine atoms are attached to the surface of the materials via CꢀF
covalent bonds. The intensity of this peak gradually increases as the FOMC fluorine
content increases.
The XPS fluorine spectra of the FOMCs are shown in Fig 5. The peaks with
8

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14, 15
binding energies of approximately 686.9 eV are due to carbonꢀfluorine groups.
again indicates the successful incorporation of fluorine into the materials.
This
The amount of fluorine in the FOMCs was determined by EDX (Energyꢀdispersive
Xꢀray 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 pꢀfluorophenol in the initial
mixture was 0.2, the fluorine content after the carbonization treatment was about 1.72
wt % and a pꢀfluorophenol 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 oneꢀpot aqueous selfꢀassembly preparation method could regulate the
4
fluorine content and avoid structural degradation if modified by Fluoride modifier.
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 FꢀMCFs which reported by
1
2
Cao . In addition, the hydrophobicity of the FOMCs increased with increasing fluorine
content, which means the hydrophobic properties of FOMC could be regulated by
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fluorine content in oneꢀpot aqueous selfꢀassembly process. The FOMCꢀ0.5 with a water
○
contact angle of 147.7 ,which is closed to the superꢀhydrophobic materials (contact
1
6
angle > 153°) .
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
oneꢀpot aqueous selfꢀassembly process using pꢀfluorophenol 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
wellꢀordered mesoporous structures comes out and exhibits its industrial applications
rely on its cheap and convenient new routine to prepare.
Acknowledgments
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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