A general approach towards efficient catalysis in Pickering emulsions stabilized by amphiphilic RGO-Silica hybrid materials

Article abstract of DOI:10.1039/c4ra07547a

A general approach towards efficient emulsion catalysis has been achieved using amphiphilic RGO-silica hybrid materials with suitable surface wettability and mesoporous structures. On the basis of the promising hybrids, a Pickering emulsion with droplets from 20-100 μm was formed and a broad range of reactions was facilitated.

Full text of DOI:10.1039/c4ra07547a

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A general approach towards efficient catalysis in
Pickering emulsions stabilized by amphiphilic
RGO–Silica hybrid materials†
Cite this: RSC Adv., 2014, 4, 35744
Received 24th July 2014
Accepted 4th August 2014
Xu-Rui Wei, Jun Liu, Yong Yang and Li Deng*a
ab
ab
ab
DOI: 10.1039/c4ra07547a
www.rsc.org/advances
A general approach towards efficient emulsion catalysis has been hydrodeoxygenation of bio-derived oxygenates in aqueous
achieved using amphiphilic RGO–silica hybrid materials with suitable phase or both phases by selective locating the active sites (Pd
surface wettability and mesoporous structures. On the basis of the nanoparticles) on the surface of the amphiphilic hybrids.
promising hybrids, a Pickering emulsion with droplets from 20–100 Another route for amphiphilic solid particles synthesis is the
mm was formed and a broad range of reactions was facilitated.
post gra using hydrophobic silanes. For instance, the octade-
cyltrichlorosilane decorated HY zeolites have been prepared for
3
d
alkylation of m-cresol and 2-propanol in a biphasic system.
Emulsion catalysis is an environmentally benign approach to
the reactions involving immiscible substrates or solvents, which
are especially attractive for water mediated organic synthesis
since most organic substrates are poorly soluble in water. In
conventional methods, surfactants have been widely employed
to emulsify the immiscible phases of an emulsion reaction
Yang et al. reported the octyl-triamine bifunctionalized silica
microspheres could support palladium for the hydrogenation of
styrene in emulsion and the as-prepared catalysts could be
4
readily recycled by the pH-triggered phase transfer. Using
carbonaceous materials whose surfaces contain oxygenated
functional groups, Zhou et al. presented the reduction of p-
nitroanisole in the pH-dependent irreversible Pickering
1
system. However, the major drawback of using surfactant
molecules lies in the difficult recovery of them from the reaction
mixture. To this end, recyclable surfactants have been devel-
oped based on polymeric materials and the Pickering emulsions
stabilized by amphiphilic solid particles have also drawn great
5
emulsions.
In this contribution, we attempt to pave a new way to
synthesize amphiphilic solid catalysts via the combination of
oleophilic graphene material and hydrophilic silica. Graphene,
2
attention. Moreover, Pickering emulsions were successfully
2
a
monolayer of sp -hydrid carbon atoms, has received
3–5
applied in emulsion catalysis in recent years. Favored by the
irreversible adsorption of solid particles at the oil–water inter-
face, Pickering emulsions are thermodynamically stable and
provide a large reaction zone for immiscible substrates. Aer
reactions the solid particles could be isolated by conventional
increasing interest from the research elds of electrodes,
supercapacitors, photoelectrics and catalysis as well. However,
the graphene materials have not gained sufficient attention in
6
emulsion catalysis, though graphene oxide (GO) has been
7
recognized as good surfactant to stabilize Pickering emulsions.
2
d,4,5

ltration, recently developed pH-induced phase inversion
We envision that the rational design of amphiphilic catalysts
with tuneable surface wettability could be realized through the
combination of desired amount of graphene and silica. To
prepare the hybrids, we choose GO and tetraethoxysilane as
starting materials to build the oleophilic moiety reduced gra-
phene oxide (RGO) and hydrophilic moiety mesoporous silica
2e
2
and CO -triggered migration. As an excellent example, Resasco
et al. fabricated the amphiphilic hybrid nanoparticles of carbon
nanotubes (CNTs) and inorganic oxides via the chemical vapor
deposition on silica and magnesia supported transition metal
3b
catalysts. Owing to the integration of the intrinsic oleophilicity
of CNTs and the hydrophilicity of inorganic oxides, the catalysts
could stabilize the biphasic reaction system and control the
(SBA-15) (Scheme 1). The advantages of this approach lie in the
following aspects: (1) GO, a relatively stable solid, is convenient
to use and to control the content in the nal hybrids, making
the synthetic method reproducible; (2) owing to the numerous
a
Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of
Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China. E-mail: oxygenate functionalities, GO can be well dispersed together
dengli@gic.ac.cn; Fax: +86 2085232917
with silane and template in water, facilitating the integration of
the two moieties; (3) GO obtained from the abundant graphite
via scalable methods could make the synthesis of the
b
University of Chinese Academy of Sciences, Beijing 100049, China
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra07547a
1
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pure SBA-15 and suggest that the hybrids could act as ideal
supports or catalysts due to the high surface area and meso-
pores. The X-ray diffraction (XRD) of SBA–G-5 exhibits a strong
ꢀ
(
100) diffraction peak at 0.90 and two weak diffraction peaks
corresponding to the (110) and (200) directions, revealing the
periodical mesoporous structure, which is consistent with the
results of N isothermal adsorption–desorption analysis.
2
In order to study the microstructure of the hybrid materials,
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) have been employed to investigate the hybrid
materials. The SEM image (Fig. 2a) shows that the SBA–G-5
consists of rod-like granules with the diameter less than1 mm,
which are aggregated into bulk phase. Besides the rod-like
granules, we also observed some curling layers distributed
randomly in the bulk phase. High resolution images (Fig. 2b
and c) reveal that the layers are similar to these of GO (Fig. S2d†)
but bear worm-like structures. These might be generated from
the deposition of the periodic ordered mesoporous silica onto
Scheme 1 Schematic diagram of the synthesis of the amphiphilic
material with mesopores.
amphiphilic materials economically feasible; (4) the porous
structures contributed by the hydrophilic moiety enable them
to accommodate both active sites and substrates. With the
RGO–silica based amphiphilic materials, we demonstrate that
they could stabilize Pickering emulsions and efficiently catalyze
a broad range of reactions, including oxidation, reduction,
hydrogenation, hydrolysis and chloromethylation.
6h
the large surface of GO during the hydrothermal process. The
electron dispersive spectrometer (EDS) reveals the RGO are
distributed in silica matrix homogeneously at mm scale
(
Fig. S4†). Aer gridding and sonicating in water, the aggre-
The GO with thickness of ca. 1 nm (Fig. S1†) was prepared
according to the modied Hummer's method. Desired amount
of the GO was homogeneously dispersed in water and then
mixed with the template P123 and HCl. Aer 2 h stirring, tet-
raethoxysilane was added to the mixture. The following
8
gated granules could be well dispersed (Fig. 2d). Moreover, the
TEM images present the ordered mesoporous channels and
hexagonal honeycomb-like cross sections, which are typical
morphology of conventional SBA-15 materials. These results
suggest that the formation of the ordered mesoporous structure
will not be signicantly inuenced by the addition of GO.
Raman spectroscopy was usually employed to evaluate the
state of carbon materials. In this study, the RGO in hybrid
synthetic steps followed the conventional method for the SBA-
9
1
5 preparation, except for the addition of excess amount of
hydrazine during the hydrothermal treatment. Aer hydro-
thermal treatment, the observation of black hydrogel suggested
the formation of 3D network structure in the presence of GO
ꢀ
and silane. Aer template removal at 600 C in Ar, the resulting
amphiphilic hybrids with colours from light gray to black were
denoted as SBA–G-x, where x referred to the weight percent of
the RGO (based on the GO loading and elemental analysis of
GO). Considering the surface activity of GO, we wondered
whether the addition of GO would inuence the P123 micelle
formation and the mesoporous structure of the nal products.
According to the N isothermal adsorption–desorption analysis
2
(Fig. 1a and S3†), the hybrid materials present typical IV
hysteresis curves, indicative of the mesoporous structure. The
2
2
BET surface areas range from 567 m to 623 m . The BJH pore
3
À1
volume and BJH average pore size are 1.14–1.16 cm g and
6.6–7.7 nm (Table S1†). These properties are similar to those of
Fig. 1 (a) N
2
isothermal adsorption–desorption isotherm of SBA–G-5; Fig. 2 Electron microscopy analysis of SBA–G-5. (a–d) SEM images;
(
b) XRD profile of SBA–G-5. (e) and (f) TEM images.
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ꢀ
material was also examined by this technique. As shown in RGO, whose contact angle is 139.2 , close to the value of pristine
À1
ꢀ
Fig. S5a,† the ratio of the D band (1344 cm ) and G band graphite (146.0 ). These values reveal that the SBA–G-5 and
(
À1
1598 cm ) intensity is ca. 0.68, which is much lower than that SBA–G-10 make good balance between the hydrophilic silica
of GO (0.97, Fig. S2a†). This suggests that the oxygenates in GO and oleophilic RGO and they may act as ideal “emulsiers” to
were reduced by hydrazine. Moreover, the X-ray photoelectron stabilize Pickering emulsions.
spectroscopy (XPS, Fig. S5b†) of SBA–G-5 shows a strong C1s
peak at the binding energy of 284.4 eV, associated with the sp
More importantly, the results also indicate that the present
method is feasible and convenient to control the surface
2
C]C species. Compared to that of GO (Fig. S2b†), the intensi- wettability of the amphiphilic.
ties of C1s peak at higher binding energy, corresponding to the
Further characterization by FT-IR (Fig. S7†) demonstrates
hydroxyl, epoxyl, carbonyl and carboxyl, are much lower, which that the intensities of the stretching vibration of hydroxyl
agree with the results of Raman spectroscopy.
decrease with the raise in RGO amount, which could be
To verify the amphiphilicity of the hybrid materials, a attributed to the enhanced surface hydrophobicity of the
comparison of the migration of pure SBA-15 and SBA–G-5 in hybrids caused by the higher loading of RGO.
water–decalin biphasic system was carried out. It is clear that
In Fig. 3c, optical microscopy images of emulsions stabilized
the hybrid material SBA–G-5 migrated to the biphasic interface, by SBA–G-5 exhibit that oil-in-water (O/W) emulsions are
whereas the SBA-15 remained in the bottom layer (Fig. S6†). stabilized by the hybrid granules and the size of emulsion
Furthermore, the surface wettability is an essential property for droplets ranges from 20 mm to 100 mm. By contrast, more
the rational design and preparation of amphiphilic materials hydrophilic SBA–G-1 and hydrophobic SBA–G-10 afford larger
suitable for stabilizing Pickering emulsions. Owing to the O/W emulsion droplets (Fig. 3b and d). Fig. 3e shows that the
porous structure, the materials quickly soaked water and dec- emulsion volume of the mixture of 4 mL water and 6 mL decalin
alin. Thus we measured the contact angles of water on the goes up until 0.08 g SBA–G-5 is added, resulting in the disap-
amphiphilic materials in decalin, instead of the air–water or pearance of the interface. Besides, the ratio of oil to water also
air–decalin contact angles. The pellets of SBA–G-x were inuences the emulsion volume. When the ratio changed from
immersed in decalin in a quartz cell and water droplets were 6 : 4 to 8 : 2, the emulsion shied from O/W to W/O type and
injected onto their surfaces. Fig. 3a shows that the hydrophilic SBA–G-5 migrated into the bottom layer (Fig. S8†). Note that the
ꢀ
SBA-15 and SBA–G-1 have small contact angles of 56.8 and as-prepared Pickering emulsions are thermodynamically stable
ꢀ
ꢀ
5
8
1
7.5 , respectively. Note that the value dramatically increased to as we observed no sedimentation or coalescence aer 1 month
2.1 when 5 wt% of RGO was mixed. For SBA–G-10, the value is stand.
ꢀ
00.7 . This could be. attributed to the strong hydrophobicity of
With the amphiphilic hybrid materials in hand, we per-
formed ve reactions (chloromethylation, hydrolysis of benzyl
chloride, reduction of nitrobenzene, hydrogenation of vanillin
and oxidative desulfurization) to testify their versatility and
efficiency for emulsion reactions. We began the catalytic tests
with the chloromethylation of toluene and hydrolysis of benzyl
chloride. These organic substrates are insoluble in aqueous
medium, so the reactions usually carried out in the presence of
10
phase transfer catalysts. As shown in Table S2,† the amphi-
philic SBA–G-5 enables the toluene to react with formaldehyde
and hydrogen chloride in aqueous phase, giving chloromethyl
toluene with a yield of 98%. By contrast, the yield is 72% in the
presence of the pure SBA-15. The reaction with surfactants PEG-
1000 and octadecyltrimethyl ammonium chloride (OTAC) could
deliver yields of 85% and 81%, respectively. Similarly, the
results of the hydrolysis of benzyl chloride (Table S3†) also
demonstrate that SBA–G-5 is an efficient phase transfer catalyst
for emulsion reaction of immiscible substrates.
The reduction of nitroaromatics to anilines is an important
industrial process. However, the commonly used reducing
2 4
agents, such as Na S, NaBH and hydrazine are aqueous soluble
5,11
while nitroaromatics are poorly soluble in water. To this end,
we performed the reduction of nitroaromatics in Pickering
emulsions stabilized by the hybrid materials. Table 1 shows that
2
only 9% nitrobenzene is reduced to aniline by Na S without
phase transfer catalyst. For the surfactants PEG-1000 and OTAC,
the yield of aniline increases to 20%. It has been reported that
graphene is an effective catalyst for the reduction of nitro-
Fig.
3 (a) Decalin–water contact angle measurements, optical
microscopy of Pickering emulsion stabilized by 1 wt%, (b) SBA–G-1, (c)
SBA–G-5 and (d) SBA–G-10 and (e) photograph of the Pickering
emulsions stabilized by SBA–G-5.
6
g
aromatics. When 5 mg RGO is added to this reaction, the yield
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Table 1 Reduction of nitroaromatics in water–decalin mixtures
transfer between the two immiscible phases and the RGO
moiety that provides the catalytic sites.
Optimized results could be achieved through varying the
volume ratio of water to decalin (Entry 7–9). 85% of aniline
could be obtained at the water : decalin ratio of 4 : 6. Further-
more, when using SBA–G-10 as catalyst, a full conversion of
nitrobenzene into aniline has been achieved. Using other
analogues of nitrobenzene as substrates (i.e. p-methylni-
trobenzene, p-methoxynitrobenzene, m-dinitrobenzene and p-
chloronitrobenzene, Entry 12–15), the yields of anilines range
from 73% to 99%.
In addition to the role of phase transfer catalysts, we believe
that the hybrid materials could act as supports to anchor metal
nanoparticles and other active phases on their large inner
surfaces. In this aspect, Pd@SBA–G-5 has been prepared for the
selective hydrogenation of vanillin, which is an important
model compound of fast pyrolysis oil and prone to distribute in
aqueous media. In principle, Pd nanoparticles lean to deposit
on the hydrophilic silica side, because the surface area is mainly
contributed by the silica matrix and its hydroxyl abundant
surface are more readily to anchor Pd nanoparticles than that of
RGO. Thus the hydrogenation of vanillin is considered to take
Catalyst loading
(mg)
Yield
(mol%)
a
Entry
Catalysts
V
water/Vdecalin
1
2
3
4
5
6
7
8
9
—
0
100
100
5
100
100
100
100
100
80
7 : 3
7 : 3
7 : 3
7 : 3
7 : 3
7 : 3
6 : 4
4 : 6
3 : 7
7 : 3
7 : 3
7 : 3
7 : 3
7 : 3
7 : 3
9
20
20
29
78
34
80
85
82
100
98
99
97
PEG-1000
OTAC
RGO
SBA–G-5
RGO + SBA-15
SBA–G-5
SBA–G-5
SBA–G-5
SBA–G-10
SBA–G-15
SBA–G-5
SBA–G-5
SBA–G-5
SBA–G-5
10
11
12
13
14
15
80
b
b
b
b
100
100
100
100
c
90
73
a
Reaction conditions: for Entry 1–11, nitrobenzene (0.31 g, 2.5 mmol)
and Na S (0.58 g, 7.4 mmol) in a water–decalin biphasic system with a place mainly in aqueous phase. On the other hand, hydrogen
2
ꢀ
b
total volume of 20 mL reacted at 80 C for 6 h. Entry 12–15, p-
methylnitrobenzene (0.25 mmol), p-methoxynitrobenzene (2.5 mmol),
m-dinitrobenzene (2.5 mmol) and p-chloronitrobenzene (2.5 mmol)
were reduced at 80 C for 8 h, respectively. Yield of m-diaminobenzene. hybrids could also facilitate the mass transfer of hydrogen gas
gas has higher solubility in organic solvents than in water, thus
the water–decalin emulsion stabilized by the amphiphilic
ꢀ
c
3b
for the hydrogenation. Table 2 shows the results of the
hydrogenation of vanillin in water–decalin emulsion stabilized
by Pd@SBA–G-5. According to the products distribution, the
amphiphilic catalyst renders the hydrogenation of vanillin to
is also improved to 29%. To our delight, the amphiphilic
SBA–G-5 enables the reaction with a high yield of 78% (Table 1,
entry 5). By contrast, the physical mixture of RGO and SBA-15
could not deliver the yield as high as the amphiphilic SBA–G-5
does (Entry 6). Based on these results, we believe that the good
performance of SBA–G-5 should be attributed to the synergic
effect of the amphiphilic surface that enhances the mass
ꢀ
vanillin alcohol (VA) at the rst hour at 90 C while 2-methoxy-4-
methyl phenol (MMP) is generated as minor product. However,
aer 4 hour reaction, most VA is converted into MMP though
hydrodeoxygenation. By comparison, both SBA-15 and SBA–G-5
showed no catalytic activity for hydrogenation due to the
absence of palladium (Entry 3 and 4). At higher temperature
Table 2 Pd@SBA–G-5 catalyzed hydrogenation of vanillin in water–decalin biphasic systems
Selectivity (%)
VA
a
ꢀ
Entry
Temperature ( C)
Reaction time (h)
Conversion of vanillin (mol%)
MMP
1
2
90
90
90
90
150
150
200
1
4
4
4
1
4
4
78
86
0
83
0
—
—
0
17
100
—
b
3
4
c
0
—
5
6
7
90
98
100
100
100
100
0
0
a
Reaction condition: vanillin (0.21 g), water (15 mL), decalin (15 mL) and Pd@SBA–G-5 (5 wt% Pd, 0.1 g) were added to a 50 mL autoclave,
b
c
vigorously stirred and reacted under 1 MPa H
2
.
Pure SBA-15 was used as catalyst. SBA–G-5 was used as catalyst.
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Table 3 Oxidative desulfurization in biphasic model systems
a
Entry
Catalysts
Catalyst loading (mg)
Conversion of DBT (mol%)
1
2
3
4
5
6
7
8
—
0
3
8
PEG-1000
SBA–G-5
80
80
20
20
80
80
80
80
80
80
80
80
80
80
33
27
35
99
92
99
98
96
93
90
94
99
99
H
3
H
3
H
3
H
3
PW12O
40
PMo12O
40
PMo12O
40@SBA–G-5
PW12O40@SBA–G-5
Cs2.5
Cs2.5
Cs2.5
Cs2.5
Cs2.5
Cs2.5
Cs2.5
Cs2.5
H
H
H
H
H
H
H
H
0.5PMo12
O
O
O
O
O
40@SBA–G-5
b
9
0.5PMo12
0.5PMo12
0.5PMo12
0.5PMo12
0.5PW12O40@SBA–G-5
0.5PMo12
40@SBA–G-5
40@SBA–G-5
40@SBA–G-5
40@SBA–G-5
b
10
11
12
13
14
15
b
b
c
O
O
40@SBA–G-5
40@SBA–G-5
d
0.5PMo12
a
ꢀ
2 2
Reaction condition: dibenzothiophene (25 mg, 0.13 mmol) in decalin (7 mL) and H O (0.5 M, 3 mL) reacted at 60 C for 3 h. Entry 9–12:
d
b
c
dibenzothiophene conversion in the 2nd–5th runs. 8 mL decalin and 2 mL water were used. 5 mL decalin and 5 mL acetonitrile were used.
ꢀ
(
150 C), Pd@SBA–G-5 allows the hydrogenation of vanillin catalyst during the ltration. By changing the ratio of water :
towards MMP with high conversion and selectivity as well. decalin or replacing water with acetonitrile, the DBT could be
ꢀ
Moreover, in all cases, even at 200 C for 4 hours, no decar- near completely removed (Entry 14 and 15). The effect of reac-
bonylation product 2-methoxylphenol has been observed. These tion time, temperature and the structure of thiophene
results suggest that the hydrogenation of vanillin could occur contaminants on this oxidative conversion have also been
selectively in aqueous phase in the presence of Pd@SBA–G-5.
Finally, the model reaction of oxidative desulfurization has
been investigated with the amphiphilic hybrids supported het-
eropoly acids (HPAs) catalyst. It was reported that the integrated
investigated (Fig. S9†).
Conclusions
catalysts containing heteropoly anions and amine cations could In summary, we have successfully prepared the RGO–silica
deeply remove dibenzothiophene (DBT), a model compound of based amphiphilic hybrid materials for efficient catalysis in
sulfur impurity in diesel, through the catalytic oxidation Pickering emulsion. These materials could be adjusted to a
12
with H O . Thus, we attempt to perform the reaction using proper wettability to act as solid emulsier and also have large
2
2
RGO–silica hybrids supported HPAs as both emulsiers and surface area to accommodate active sites and substrates to
catalysts. In Table 3, it is clear that the autocatalytic oxidation is facilitate catalytic conversions. The excellent performance in
not sufficient to remove DBT (Entry 1 and 2). The amphiphilic ve classes of reactions highlights the superiority and feasibility
hybrid SBA–G-5 could convert 33% DBT due to the emulsion of these amphiphilic catalysts based on the RGO–silica hybrids.
effect. It is noteworthy that the H
3
PMo12O40@SBA–G-5 gives
much better performance than that using H PMo O or
3
12 40
Acknowledgements
H PW O solely (Entry 4–6). In the presence of H PMo -
3
12 40
3
12
The authors are grateful to Natural Science Foundation of China
O @SBA–G-5, the sulfur content could be reduced to less than
4
0
(21302230) and Youth Innovation Promotion Assosiation, CAS
3
10 ppm. For H PW12O40@SBA–G-5, the conversion of DBT can
for the nancial support.
also reach 92%. Regarding the leaching of HPAs from the
catalysts in water, we deposit the Cs substituted HPAs onto
the surface of SBA–G-5 for this reaction. Consequently, Notes and references
the conversion of DBT is 99% and 94% in the presence of
1
(a) C. M. Starks, J. Am. Chem. Soc., 1971, 93, 195; (b)
A. W. Herriott and D. Picker, J. Am. Chem. Soc., 1975, 97,
Cs2.5H0.5PMo O @SBA–G-5 and Cs2.5H0.5PW O @SBA–G-5,
12
40
12 40
respectively. For Cs2.5H0.5PMo O @SBA–G-5, no obvious
12
40
2
4
345; (c) E. V. Dehmlow, Angew. Chem., Int. Ed., 1977, 16,
93; (d) H. H. Freedman, Pure Appl. Chem., 1986, 58, 857.
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