PH-Responsive Pickering emulsion stabilized by polymer-coated silica nanoaggregates and applied to recyclable interfacial catalysis

Article abstract of DOI:10.1039/d0ra07957j

We first synthesized a diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-(trimethoxysilyl)propyl methacrylate] (PtBMA-b-PTMSPMA) through reversible addition-fragmentation chain transfer (RAFT) living radical polymerization and grafted it onto fumed silica by converting the PTMSPMA segment to silanol and the PtBMA segment to polymethylacrylic acid (PMAA) in the presence of trifluoroacetic acid in order to obtain PMAA brush-coated silica nanoaggregates P-Si. TEM, DLS, FTIR, and TGA results confirmed the successful modification of the starting materials. The nanoaggregates flocculated and stabilized a toluene-in-water Pickering emulsion at low pH, while the nanoaggregates were well dispersed in water and broke the emulsion under both neutral and basic conditions. Alternatively, the addition of acid/base induced emulsification/demulsification cycles that were sustained for several cycles. Moreover, when the P-Si was mixed with Rh-loaded silica, Rh-Si, the mixture had the same pH-responsive Pickering emulsion behavior as the single P-Si. This Pickering emulsion system can be used in the biphasic interfacial catalytic hydrogenation of olefins and had excellent yields under a hydrogen atmosphere. The yield of Pickering emulsion catalysis rapidly reached more than 99% in 3 h, while that of the demulsified mixture failed to reach 20% in 4 h, which verified the promotion of catalysis by the Pickering emulsion. Base-induced demulsification can be used to separate the products and recycle the catalyst. This pH-responsive Pickering emulsion catalytic system was capable of several cycles of reuse, and there was no significant decrease in catalytic efficiency even after eight cycles. This journal is

Full text of DOI:10.1039/d0ra07957j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
pH-Responsive Pickering emulsion stabilized by
polymer-coated silica nanoaggregates and applied
to recyclable interfacial catalysis†
Cite this: RSC Adv., 2020, 10, 42423
Ruidong Luo,
Jinfeng Dong and Yunbai Luo
*
We first synthesized a diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-(trimethoxysilyl)propyl
methacrylate] (PtBMA-b-PTMSPMA) through reversible addition–fragmentation chain transfer (RAFT)
living radical polymerization and grafted it onto fumed silica by converting the PTMSPMA segment to
silanol and the PtBMA segment to polymethylacrylic acid (PMAA) in the presence of trifluoroacetic acid
in order to obtain PMAA brush-coated silica nanoaggregates P-Si. TEM, DLS, FTIR, and TGA results
confirmed the successful modification of the starting materials. The nanoaggregates flocculated and
stabilized a toluene-in-water Pickering emulsion at low pH, while the nanoaggregates were well
dispersed in water and broke the emulsion under both neutral and basic conditions. Alternatively, the
addition of acid/base induced emulsification/demulsification cycles that were sustained for several
cycles. Moreover, when the P-Si was mixed with Rh-loaded silica, Rh-Si, the mixture had the same pH-
responsive Pickering emulsion behavior as the single P-Si. This Pickering emulsion system can be used in
the biphasic interfacial catalytic hydrogenation of olefins and had excellent yields under a hydrogen
atmosphere. The yield of Pickering emulsion catalysis rapidly reached more than 99% in 3 h, while that
of the demulsified mixture failed to reach 20% in 4 h, which verified the promotion of catalysis by the
Pickering emulsion. Base-induced demulsification can be used to separate the products and recycle the
catalyst. This pH-responsive Pickering emulsion catalytic system was capable of several cycles of reuse,
and there was no significant decrease in catalytic efficiency even after eight cycles.
Received 17th September 2020
Accepted 16th November 2020
DOI: 10.1039/d0ra07957j
rsc.li/rsc-advances
30–36
double stimuli,
have been used to induce the demulsica-
1
Introduction
tion or phase inversion of Pickering emulsions. Manipulating
Pickering emulsions are biphasic systems in which droplets are the pH by alternately adding acid or base to the system is simple
stabilized by nanoparticles instead of environmentally harmful and easy to implement. Though salts are generated and will
1,2
surfactants. Because of the irreversible adsorption of nano- accumulate in the system, these are not harmful to the envi-
3
particles at the oil–water interface, Pickering emulsions are ronment. Moreover, compared to pH-responsive Pickering
highly stable and offer many opportunities for a variety of emulsions, modifying nanoparticles in order to obtain other
4
5
applications, such as multifunctional materials, oil recovery,
etc. In some applications, such as Pickering emulsion interfa- not that common.
cial catalysis and controllable encapsulation and release of
Yang was the rst to use these stimuli-responsive properties
stimuli-responsive Pickering emulsions is too complicated and
37
cargoes, a permanent and stable Pickering emulsion is not to promote Pickering emulsion interfacial catalysis. In the
needed, and breaking the emulsion is complicated and energy- latter, the formation of the emulsion increases the oil–water
consuming. Thus, using an easy means of breaking the emul- contact area, and thus accelerates the reaction rate of biphasic
sion or reversing the phase of the emulsion, such as an outer catalysis. However, following the reaction, separation of the
stimulus, is important.
catalytic products and recycling of the catalyst are also needed.
38
Stimuli-responsive Pickering emulsions have been well Thus, stimuli-responsive Pickering emulsions offer a way to break
6
developed in recent decades, and many stimuli, including CO / or reverse the emulsion in order to make the above goals easier to
2
7
–11
12–19
20–25
26
27–29
37,39–41
42
29
N ,
2
pH,
temperature,
redox reduction, light,
and achieve. Many stimuli, including pH,
CO
2
/N
2
,
light,
43
44
temperature, and double stimuli, have been used to isolate the
product and recycle the catalyst. Among these, varying the pH is
rapid and easy to manipulate, and thus an attractive method for
designing a recyclable Pickering emulsion catalyst system.
In this study, we have built a pH-responsive Pickering
emulsion system that was stabilized by polymer-coated silica
Engineering Research Center of Organosilicon Compounds & Materials, Ministry of
Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan
4
†
1
30072, PR China. E-mail: ybai@whu.edu.cn
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra07957j
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 42423–42431 | 42423

View Article Online
RSC Advances
Paper
nanoaggregates, P-Si, and the properties of the emulsion were
2.2.2 Synthesis of the diblock copolymer PtBMA-b-
fully characterized. The nanoaggregates occulated at low pH PTMSPMA. PtBMA macro-CTA (11.5 g, 1.07 mmol), TMSPMA
values and were able to stabilize the Pickering emulsion, while (3.81 g, 15.36 mmol), AIBN (0.037 g, 0.23 mmol), and CHCl (50 mL)
3
they dispersed in water under neutral and basic conditions and were added to a Schlenk tube and bubbled with N gas for more
2
prevented the emulsion from forming. Thus, the system could than 20 min to expel the air. The reaction was conducted at
ꢀ
be switched from emulsication to demulsication by alter- 60 C for 12 h. The nal red powder product, PtBMA-b-
nately adding acid or base to the system many times. Moreover, PTMSPMA, was puried by being precipitated three times from
fumed silica-loaded Rh had the same pH-responsive behavior a mixture of methanol and water (v/v ¼ 7/3).
when mixed with the nanoaggregates P-Si, even though it was
unable to stabilize the emulsion alone. The Pickering system,
2.3 Synthesis of polymer-coated silica nanoaggregates
which greatly increased the oil–solid contact area, could be used
for the hydrogenation of alkenes when the reactant was added
to the oil phase and hydrogen was bubbled through the system.
The system was demulsied and separated into two phases with
the addition of base, while the oil phase, containing product,
could be isolated and the aqueous phase, containing nano-
catalyst, could be recycled for further Pickering emulsion
catalysis. Thus, these experiments have conrmed that pH-
responsive Pickering emulsions stabilized by polymer-coated
silica nanoaggregates provide a rapid means of recovering
catalytic products and catalysts, and might therefore nd wider
application in the future.
Fumed silica was coated with the PtBMA-b-PTMSPMA polymer
in toluene using TFA as catalyst.
Typically, 2.0 g fumed silica was ultrasonicated for 5 min in
2
0
00 mL toluene to disperse the nanoparticles. Subsequently,
.5 g PtBMA-b-PTMSPMA was added under stirring. Aer stir-
ring for 30 min, 2 mL TFA was added to catalyze the reaction.
During the reaction, the PTMSPMA segment was changed to
silanol by acidolysis of TFA and then reacted and graed onto
the fumed silica. The PtBMA segment changed to poly-
methylacrylic acid (PMAA) by acidolysis of TFA. Thus, PMAA-
coated silica was nally achieved. The particles were made to
settle out by means of centrifugation at 12 000 rpm for 10 min
and puried by being redispersed in ethanol and settled out
again by centrifugation. This dispersion–centrifugation–
dispersion procedure was repeated three times to remove any
2
Experimental
2.1 Materials
excess polymer. The nal product (pink powder) was dried
ꢀ
Fumed silica (HL-200) with a primary diameter of approximately under vacuum at 120 C for 2 h and denoted as P-Si; the
2
0 nm was provided by Guangzhou GBS High-Tech and Industry synthesis of P-Si is shown in Scheme 1.
Co., Ltd (Guangzhou, China). The RAFT agent 2-cyano-2-propyl
benzodithioate (CPDB) was synthesized according to the
2
.4 Synthesis of Rh-loaded silica
45
procedure described in the literature. The monomer tert-butyl
methacrylate (tBMA, 99%) and 3-(trimethoxysilyl)propyl meth-
acrylate (TMSPMA, 97%) were purchased from Aladdin Indus-
Rh-loaded silica was obtained by wet reduction of Rh(III). Typi-
cally, 0.5 g fumed silica was well dispersed in 50 mL water by
ultrasonic treatment. Then, RhCl $3H O (0.0128 g, 0.048 mmol)
0
3
2
trial Corporation (Shanghai, China). The initiating agent, 2,2 -
was added and the mixture stirred for 4 h. Next, aqueous NaBH
4
azobis(2-methylpropionitrile) (AIBN), was recrystallized twice
from ethanol before use. Toluene (99.5%) was purchased from
Aladdin. Triuoroacetic acid (TFA, 99%) and rhenium tri-
solution (0.0073 g, 0.193 mmol, dissolved in 5 mL water) was
added dropwise to the dispersion under rapid stirring and reacted
overnight. Following the reaction, the resulting black powder was
isolated by means of centrifugation and was washed with water
four times to ensure no small molecules remained. The nal
loading of Rh was 0.88 wt% for the mixture as determined by
inductively coupled plasma atomic emission spectroscopy (ICP-
AES), and this Rh-loaded silica was denoted as Rh-Si.
3 2
chloride hydrate (RhCl $3H O, 99%) was purchased from
Energy-chemical Corporation (Shanghai, China). Sodium
borohydride (NaBH4, 96%) was purchased from Sinopharm
Group Co., Ltd (Beijing, China). Ultrapure water was generated
from a Milli-Q® Ultrapure Water System.
Moreover, different amount of RhCl
3 2
$3H O were used here,
and the corresponding loading of Rh were 0.41 wt% and
2
.2 Synthesis of the PtBMA-b-PTMSPMA diblock copolymer
The diblock copolymer poly[tert-butyl methacrylate]-b-poly[3-
trimethoxysilyl)propyl methacrylate] (PtBMA-b-PTMSPMA) was
1.82 wt%.
(
2.5 Characterization of the polymer and nanoparticles
synthesized by a typical RAFT living radical polymerization, as
described below.
2.5.1 Characterization of the polymer. The diblock copol-
1
2
.2.1 Synthesis of PtBMA macro-CTA. tBMA (14.2 g, 100 mmol), ymer was characterized by H NMR (400 MHz Bruker Avance-
CPDB (0.221 g, 1 mmol), AIBN (0.041 g, 0.25 mmol), and CHCl
3
400 spectrometer, Bruker BioSpin Corporation, Billerica, MA,
(
40 mL) were added to a Schlenk tube and bubbled with N
2
gas for USA) and gel permeation chromatography (GPC) measurements
more than 20 min to expel the air. The reaction was conducted at (Wyatt GPC instruments, Wyatt Technology Corporation, Santa
ꢀ
6
0 C for 12 h. The nal red powder, PtBMA macro-CTA was puried Barbara, CA, USA). The corresponding gures are shown in the
by being precipitated three times from a mixture of methanol and ESI† and the diblock copolymer can be expressed as PtBMA74-b-
water (v/v ¼ 7/3). PTMSPMA17 based on the characterization.
42424 | RSC Adv., 2020, 10, 42423–42431
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Scheme 1 Synthesis of the diblock copolymer PtBMA-b-PTMSPMA and P-Si.
2.5.2 Dynamic light scattering (DLS). The hydrodynamic
2.5.4 Transmission electron microscopy (TEM). TEM
diameters of the fumed silica, P-Si, and Rh-Si and the zeta potential images of fumed silica, PMAA-Si, and Rh-Si were obtained on
of the P-Si at different pH values were measured using a Malvern a JEM-2100 transmission electron microscope (JEOL, Tokyo, Japan).
Zetasizer Nano ZS (Malvern Panalytical Ltd, Malvern, UK).
.5.3 Thermogravimetric analysis (TGA). TGA of the fumed and P-Si were recorded on a Thermo Scientic Fourier Infrared
silica and P-Si was performed on a NETZSCH TG 209 F1 spectrometer (Thermo Scientic, Waltham, MA, USA).
instrument (NETZSCH TAURUS Instruments GmbH, Weimar, 2.5.6 Surface tension (SFT) and interfacial tension (IFT).
2.5.5 Infrared spectrum (IR). The IR spectra of fumed silica
2
Germany) using air as the heating atmosphere. A typical The SFT of a P-Si dispersion (1.0 wt%) at different pH values was
measurement involved heating a sample from room tempera- measured using the plate method on a Kr u¨ ss K100 surface tension
ꢀ
ꢀ
ꢁ1
¨
ture to 800 C at a rate of 10 C min
.
meter (KRUSS GmbH, Hamburg, Germany). The IFT between the
Fig. 1 (a) A TEM image of fumed silica, the scale bar: 100 nm; (b) a TEM image of P-Si, the scale bar: 500 nm; (c) DLS results of fumed silica and P-
Si; (d) FTIR results of the diblock copolymer, fumed silica and P-Si; (e) TG results of fumed silica, P-Si and the diblock copolymer PtBMA74-b-
PTMSPMA17
.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 42423–42431 | 42425

View Article Online
RSC Advances
Paper
Fig. 2 (a) Images of P-Si aqueous dispersions at different pH values; (b) cycle of flocculation and dispersion of P-Si under the stimulus of acids/
bases; (c) zeta potential of P-Si at different pH values.
same dispersion and toluene was measured using the pendant
3
Results and discussion
drop method on a Kr u¨ ss DSA100 drop shape analyzer.
.5.7 Contact angle. The contact angles of water (pH 3 and
) on the nanoparticles were measured using the sessile drop To get a better understanding of P-Si, the raw material (fumed
3.1 Characterization of P-Si
2
9
method on a Kr u¨ ss DSA100 drop shape analyzer.
silica, HL-200) was characterized by TEM and DLS measure-
ments (Fig. 1a and c). From the gure, it can be seen that the
primary nanoparticles of fumed silica were approximately
2.6 Preparation and characterization of Pickering emulsions
20 nm in size, but there were no clear boundaries between the
The nanoparticle aqueous dispersions (different concentra-
tions, different pH values) were mixed with a toluene phase of
the same volume, and then the mixture was emulsied by
means of vigorous hand shaking or homogenization
nanoparticles. The DLS results revealed that the hydrodiameter
of the fumed silica was 188 nm (polydispersity index (PDI) ¼
0.104), which was much greater than the diameter of the
primary nanoparticles. Thus, fumed silica nanoparticles
became nanoaggregates because of their high surface energy,
and the basic units of fumed silica that dispersed in water or
were chemically modied were actually nanoaggregates. Thus,
when the fumed silica was coated with the diblock copolymer
PtBMA-b-PTMSPMA using the sol–gel method, the basic units
that were chemically modied were the nanoaggregates, not the
primary nanoparticles. As shown in Fig. 1b, the TEM sample
was prepared using 0.01 wt% P-Si aqueous dispersion. Thus, the
basic constituent units of the P-Si were the various-sized
branched nanoaggregates of the primary nanoparticles. The
DLS, FTIR, and TGA results conrmed the successful modi-
cation of the polymer on the nanoaggregates. As shown in
Fig. 1c, the hydrodynamic diameter of the P-Si was about
(
13 500 rpm, 2 min) using an IKA ULTRA-TURRAX T 25
homogenizer (IKA® England Ltd, Oxford, UK). The emulsion
droplets were observed and recorded using an Olympus BX51
microscope (Olympus Corporation, Tokyo, Japan), and the
emulsion type was determined by means of a drop test.
2
.7 Performance of the Pickering emulsion catalysis in the
catalysis of hydrogenation
P-Si and Rh-Si (m/m ¼ 1 : 1) were mixed together and dispersed
into 2 mL water. The total concentration was set at 1.0 wt%, and
NaOH and HCl was used to adjust the pH. A small volume of the
toluene phase (2 mL) dissolved in an olen (1 mmol) was used
as the oil phase. A hydrogen atmosphere was provided by
a balloon of hydrogen gas. Gas chromatography (GC-2010 Plus,
Shimadzu, Kyoto, Japan) was used to measure the yield.
205 nm (PDI ¼ 0.132), a little larger than that of the fumed silica,
which indicated that the polymer brush on the nanoaggregates
Fig. 3 Pickering emulsions prepared between PMAA-Si aqueous dispersion (1.0 wt%, different pH values, 2 mL) and toluene (2 mL). Emulsifying
method: (a) vigorous hand-shaking; (b) homogenizing using an IKA ULTRA-TURRAX T 25 homogenizer (13 500 rpm, 2 min); (c) emulsion
dropped into oil; (d) emulsion dropped into water.
42426 | RSC Adv., 2020, 10, 42423–42431
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
increased the diameter. In the FTIR spectra of the polymer, the
fumed silica, and the P-Si shown in Fig. 1d, the P-Si showed not
only the characteristic peaks of silica but also those of the polymer:
ꢁ
1
ꢁ1
the bands at 2833–3040 cm (C–H) and 1720 cm (C]O) in the
spectrum of PMAA-Si conrmed the successful modication. The
TGA results in Fig. 1e indicate that the fumed silica did not show
any apparent loss in weight, and TG curve of the diblock copolymer
had a similar shape with that of the P-Si.
So the weight loss of the P-Si (about 19.24%) was belonged to
the polymer graed on the nanoaggregates.
The polymer brush on the P-Si was PMAA, which is a pH-
responsive polymer. Thus, the effect of pH on the dispersion
behavior of the P-Si in water was taken into consideration, as
Fig. 4 (a) Cycles of emulsification and demulsification of PMAA-Si
under the stimulus of acid/base; (b) image of Pickering emulsion
shown in Fig. 2a. The P-Si was well dispersed in water when the droplets in the first cycle (scale bar: 100 mm); (c) image of Pickering
emulsion droplets in the fourth cycle (scale bar: 100 mm).
pH was above 4, and the P-Si occulated at the bottom of the
aqueous phase when the pH was no more than 4. Fig. 2c shows
that the zeta potential of the P-Si was mostly between ꢁ30 and
ꢁ
40 mV when the pH was above 4, and so the nanoaggregates phase still contained dispersed nanoparticles at pH 4, so the P-Si
were able to be dispersed in water because of the electrostatic did not fully adsorb at the toluene–water interface to stabilize the
repulsion between them. The zeta potential approached 0 mV as emulsion. While the remaining aqueous phase was clear at pH 2
the pH decreased, so that the P-Si occulated. Moreover, Fig. 2b and 3, all of the nanoparticles were used to stabilize the emulsion.
reveals that the P-Si was able to transform between the occu-
A different set of emulsication results resulted when the
lated state and the dispersion state by alternately injecting acid mixture was emulsied using the homogenizer: the Pickering
or base several times to the dispersion. The pH-responsive emulsion formed at pH 6–8, and the nanoparticles were dispersed
in the corresponding remaining aqueous phase. The difference
dispersion behavior and the change in zeta potential of the P-
Si was due to the pH-responsive protonation of the PMAA brush. between the emulsication results might have resulted from the
When the nanoparticles were able to stabilize the Pickering fact that the high-speed shearing probe of the homogenizer shat-
emulsion, the nanoparticles were amphiphilic and able to be tered the nanoaggregates into smaller pieces through the shearing
adsorbed at the oil–water interface. As the results of the contact force. Thus, in order to maintain the integrity of the P-Si nano-
angle test shown in Fig. S2a and b† suggest, the sessile drop at aggregates, vigorous hand-shaking was adopted as the emulsifying
different pH values had different contact angles. The contact method in the following experiments. Moreover, the type of Pick-
ꢀ
angle at pH 3 was 46 , which meant that the P-Si was amphi- ering emulsion was oil-in-water, as determined by a drop test
Fig. 3c and d). An emulsion drop spread apart on water and
(
philic but biased toward being hydrophilic. The contact angle at
ꢀ
pH 9 was 7 , which meant that the P-Si was extremely hydro- remained as a white drop on oil, which indicated that the external
philic. The SFT and IFT data shown in Fig. S2† characterize the phase of the Pickering emulsion was water.
absorption of the P-Si at the surface of the aqueous dispersion
and the toluene–water interface. The SFT and IFT values at pH above. Thus, the effect of the concentration of the P-Si on the
A pH value of 3 was good for emulsication, as mentioned
ꢁ1
values above 7 were about 71 and 30 mN m , respectively, Pickering emulsion was taken into consideration when the pH
which were close to the SFT value of pure water and the IFT was set to 3 and hand-shaking was used as the emulsifying
value of the pure water–toluene interface, respectively. Thus, the method. As shown in Fig. S3a,† the Pickering emulsion formed
at P-Si concentrations of 0.1–3.0 wt%, and the volume of the
P-Si nearly failed to adsorb at the surface and the interface under
these conditions. As the pH decreased, the SFT and IFT values
ꢁ
1
decreased about 15 mN m , which indicated that the P-Si was
able to be adsorbed at the surface and the toluene–water interface
at low pH. The contact angle, SFT, and IFT results all displayed the
amphiphilicity of the P-Si and the possibility of the P-Si stabilizing
Pickering emulsions at low pH values.
3.2 Characterization of the Pickering emulsion and its pH-
responsive behavior
As has been described above, the P-Si showed pH-responsive
dispersion behavior. Therefore, the emulsifying ability of the
P-Si as a function of pH was tested using two different methods.
As shown in Fig. 3, using vigorous hand-shaking as the rst
emulsifying method, the Pickering emulsion formed only at pH
values no greater than 4. Furthermore, the remaining aqueous Fig. 5 A TEM image of the Rh-Si (scale bar: 100 nm).
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 42423–42431 | 42427

View Article Online
RSC Advances
Paper
then added to the emulsion to adjust the pH to approximately 7
with continued shaking. The emulsion rapidly and completely
separated into two phases within a minute, and the emulsion
droplets that had adhered to the wall of the vial disappeared.
Subsequently, HCl was added to the dispersion to adjust the pH to
3, and a Pickering emulsion formed again with vigorous hand-
shaking. These cycles of emulsication and demulsication of
the Pickering emulsion by alternately adding acid or base could be
sustained for several cycles. As shown in Fig. 4b and c, there was no
signicant change in the diameter of the emulsion droplets, thus
displaying excellent pH-responsive behavior.
3.3 Enhanced catalysis of hydrogenation by the Pickering
Fig. 6 (a) Cycles of emulsification and demulsification of a mixture of emulsion
P-Si and Rh-Si under the stimulus of acid/base; (b) image of Pickering
emulsion droplets in the first cycle (scale bar: 100 mm); (c) image of
Pickering emulsion droplets in the fourth cycle (scale bar: 100 mm).
To apply P-Si to Pickering emulsion catalysis, we rst synthe-
sized the Rh-loaded fumed silica, Rh-Si. A TEM image of the Rh-
Si (loading of Rh was 0.88 wt%) is shown in Fig. 5. It can be seen
that the black particles are Rh and the gray particles are the
fumed silica. Thus, Rh was successfully doped and mixed into
the fumed silica. When measuring the emulsifying ability of the
Rh-Si, it was found that the Rh-Si was unable to stabilize the
emulsion at any pH value, while the mixture of P-Si and Rh-Si
Pickering emulsion increased as the concentration of the P-Si
increased. When determining the statistics of the diameter
distribution of droplets on the images of Pickering emulsion
droplets using the GuassAmp function, as shown in Fig. S3b
and c,† the average diameter of Pickering emulsion droplets
decreased as the concentration of the P-Si increased, and the
distribution of diameters was correspondingly narrower. The
diameter of Pickering emulsion droplets decreased rapidly
when the concentration was lower than 1.25 wt% and decreased
only slightly when the concentration was higher; the diameters
of Pickering emulsion droplets were in the range 15–30 mm
when the concentration of the P-Si was more than 1.5 wt%.
The pH-responsive behavior of the P-Si-stabilized Pickering
emulsion is shown in Fig. 4a. The P-Si was dispersed in 2 mL water
to get a concentration of 1.0 wt%, and the pH was adjusted to 3
with HCl to occulate the nanoaggregates. An equal volume of
toluene was added, and the mixture was emulsied by means of
vigorous hand-shaking to form a Pickering emulsion. NaOH was
(
m/m ¼ 1 : 1) had the same pH-responsive emulsifying
behavior as the single P-Si. The pH-responsive behavior of
a mixture of P-Si and Rh-Si (m/m ¼ 1 : 1, total concentration of
1.0 wt%) is shown in Fig. 6a. When adjusting the pH to
approximately 7 by adding NaOH, the system separated into two
phases, while the upper layer was toluene and the lower black
aqueous layer was a dispersion of P-Si and Rh-Si. Subsequently,
the addition of HCl to adjust the pH to 3 and emulsifying the
mixture by means of hand-shaking, a Pickering emulsion
formed. From the images of this emulsion, the emulsion layer
was found to be gray and the lower remaining aqueous phase
was found to be transparent and colorless. Thus, all of the Rh-Si
was located in the emulsion layer, and this would be helpful for
Pickering emulsion catalysis. However, the diameters of the
Fig. 7 (a) Enhanced catalysis of hydrogenation of lauryl methacrylate by the Pickering emulsion; (b) the yield of the Pickering emulsion catalysis
and demulsified biphases at different times: the loading of Rh in Rh-Si was 0.88 wt% and the concentration of the mixture was 1.0 wt%; (c) the
yield of Pickering emulsion catalysis over multiple catalytic cycles in 3 h.
42428 | RSC Adv., 2020, 10, 42423–42431
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
and thus the contact between the Rh-Si and the olen in
the toluene phase was greatly promoted. This system, in
which P-Si was used to form Pickering emulsion and Rh-Si
acted as catalyst, could enhance the catalysis of the hydro-
genation of olens under a hydrogen atmosphere. To verify
this assumption, the catalysis of the hydrogenation of lauryl
methacrylate was set as the model reaction, as shown in
Fig. 7a. Two states of the mixture shown in Fig. 7 were used to
compare the yield of the catalytic reaction as a function of
time: the aqueous phase (2 mL) was a dispersion of a mixture
of P-Si and Rh-Si, a hydrogen atmosphere was provided by
means of a balloon of hydrogen gas, 1 mmol lauryl methacrylate
was dissolved in 2 mL toluene, and the yield of the catalysis was
determined by gas chromatography (GC). The results are shown in
Fig. 6b. For the Pickering emulsion, the yield quickly increased to
Scheme 2 Catalytic cycles of a pH-responsive Pickering emulsion.
4
3% in 0.5 h and reached more than 99% within 3 h. By contrast,
the yield increased slowly and reached no more than 20% within
h in the case of the demulsied mixture. The comparison of the
Pickering emulsion droplets stabilized by the mixture were
slightly larger than those of the single P-Si (average diameters:
4
87 mm for the mixture and 67 mm for the single P-Si, the larger
yields proved the catalytic enhancement by the Pickering emul-
sion, which resulted from the much larger area of the Rh-Si
compared to the oil.
size of Pickering emulsion droplet was the result of the lower
concentration of the effective emulsifying component – the P-
Si). On adding NaOH to adjust the pH to 7 and shaking the
vial for a while, the Pickering emulsion completely separated
into two phases. Further addition of HCl to adjust the pH to 3
and emulsifying the mixture by means of hand-shaking led to
the formation of a Pickering emulsion again. This pH-
responsive behavior was sustained over several cycles. Fig. 6b
and c shows that there were no signicant changes in the
Pickering emulsion droplets aer several cycles of emulsica-
tion and demulsication. The reason why the Rh-Si was able to
stabilize the emulsion together with the P-Si might be because
the fumed silica can easily become aggregates, so that the Rh-Si
As was mentioned above in Fig. 6a, the Pickering emulsion
displayed
pH-induced
emulsication/demulsication
behavior. This property could be used to isolate a product
and recycle the catalyst several times. As shown in Scheme 2,
P-Si and Rh-Si co-stabilized the Pickering emulsion at pH 3
and were used to catalyze the hydrogenation of lauryl meth-
acrylate under a hydrogen atmosphere. NaOH was added to
adjust the pH to 7, and the emulsion completely separated
into two phases, the upper toluene phase with the product
being isolated through suction by a straw and the yield being
determined by GC. A new toluene phase dissolved in lauryl
methacrylate was added to initiate a new catalytic cycle. The
results for several catalytic cycles are shown in Fig. 7c; the
high activity of the Pickering emulsion catalyst was
conrmed by a yield of greater than 95%, even aer eight
cycles. In summary, the pH-responsive Pickering emulsion
exhibited an excellent effect on the recycling of catalyst and
the isolation of product.

occulated together with the P-Si at pH 3, enabling the Rh-Si to
adhere to the surface of the emulsion droplets, which were
coated with the P-Si.
The Pickering emulsion was stabilized by nanoparticles,
which thus greatly increased the contact area of the oil–water
and oil–solid interfaces, which is benecial for biphasic
catalysis. In this Pickering emulsion system, the Rh-Si
stabilized the Pickering emulsion together with the P-Si,
Fig. 8 (a) Images of Pickering emulsion droplets (scale bar: 100 mm) stabilized by different concentration of a mixture of P-Si and Rh-Si (m/m ¼
1
0
: 1); (b) the yield of Pickering emulsion catalysis at different times: the loading of Rh in Rh-Si was 0.88 wt% and the concentration varied from
.5 wt% to 3.0 wt%; (c) the yield of Pickering emulsion catalysis at different times: the concentration of the mixture was 1.0 wt% and the loading of
Rh varied from 0.41 wt% to 1.82 wt%.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 42423–42431 | 42429

View Article Online
RSC Advances
Paper
Table 1 Results of catalytic hydrogenation by means of the Pickering concentration of the mixture was 1.0 wt%), which is suitable for
emulsion
the catalytic hydrogenation of a wide range of olens.
Substrate
Product
t (h)
Yield (%)
$99
4
Conclusions
3
In this study, we have built a pH-responsive Pickering emulsion
system stabilized by polymer-coated nanoaggregates, P-Si. The
nanoaggregates occulate and stabilize the toluene-in-water
Pickering emulsion at low pH values, while the nano-
aggregates are well dispersed in water and break the emulsion
under neutral and basic conditions. These acid/base-induced
emulsication/demulsication cycles can be sustained over
several cycles. Moreover, when the P-Si is mixed with Rh-loaded
silica, Rh-Si, the mixture has the same pH-responsive behavior
as the single P-Si. This system can be used in the biphasic
interfacial catalytic hydrogenation of olens, and achieves
yields of $99% under a hydrogen atmosphere. Base-induced
demulsication can be used to separate the products and
recycle the catalyst. This pH-responsive Pickering emulsion
catalytic system can operate over multiple cycles, and there is no
signicant decrease in catalytic efficiency.
2
2
.5
$99
$99
2
2
3
$99
$99
$99
.5
.5
2
$99
Conflicts of interest
The concentration of the mixture of P-Si and Rh-Si and the
loading of Rh in the Rh-Si were also taken into consideration. As
shown in Fig. 8a and b, the size of Pickering emulsion droplets
decreased as the concentration of the mixture went up, the
corresponding speed of Pickering emulsion catalysis was
accelerated. More specically, the yield of hydrogenation of
lauryl methacrylate reached more than 99% within 1 h when the
concentration was 3.0 wt%, while the time needed to complete
the catalysis was 5 h when the concentration was 0.5 wt%. The
reason would be that the smaller Pickering emulsion droplets
increased the contact area of oil phase and Rh-Si and thus
accelerated the Pickering emulsion catalysis. As shown in
Fig. 8c, different loading of Rh in Rh-Si was adopted in the
Pickering emulsion catalysis, from the result, it could be found
that high loading of Rh would increased the speed of catalysis
but the degree of improvement was not as obvious as the
decrease of the size of emulsion droplets. Specically, the yield
reached more than 99% within 2.5 h when the loading of Rh
was 1.82 wt%, while time needed to complete the catalysis was
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC 21573164 and 21773174).
References
1
2
3
4
5
S. U. Pickering, J. Chem. Soc., Trans., 1907, 91, 2001.
W. Ramsden, Proc. R. Soc. London, 1903, 72, 156.
B. P. Binks, Curr. Opin. Colloid Interface Sci., 2002, 7, 21–41.
D. Yin, L. Ma, J. Liu and Q. Zhang, Energy, 2014, 64, 575–581.
K. Y. Yoon, H. A. Son, S. K. Choi, J. W. Kim, W. M. Sung and
H. T. Kim, Energy Fuels, 2016, 30, 2628–2635.
6
7
8
9
J. Tang, P. J. Quinlan and K. C. Tam, So Matter, 2015, 11,
3512–3529.
4
h when the loading of Rh was 0.41 wt%. In summary, for
X. Yan, Z. Zhai, J. Xu, Z. Song, S. Shang and X. Rao, J. Agric.
Food Chem., 2018, 66, 10769–10776.
Y. Shi, D. Xiong, Z. Li, H. Wang, Y. Pei, Y. Chen and J. Wang,
ACS Sustainable Chem. Eng., 2018, 6, 15383–15390.
J. Jiang, Y. Zhu, Z. Cui and B. P. Binks, Angew. Chem., 2013,
Pickering emulsion catalysis, the key to increase the catalytic
rate was to decrease the size of emulsion droplets to increase
the contact area; increasing the loading of Rh would increased
the speed of catalysis but to a limited degree.
In addition, different kinds of olens were adopted in this
Pickering emulsion system to verify its wide applicability.
Catalytic hydrogenation could run very well with different kinds
of olens, and the products are easily isolated. As shown in
Table 1, different kinds of olens, including styrene monomers,
methacrylate monomers, and acrylamide monomers, had yields
of more than 99% within 3 h in this Pickering emulsion catalytic
system (the loading of Rh in Rh-Si was 0.88 wt% and the
5
2, 12373–12376.
0 C. Liang, Q. Liu and Z. Xu, ACS Appl. Mater. Interfaces, 2014,
, 6898–6904.
1 Y. Qian, Q. Zhang, X. Qiu and S. Zhu, Green Chem., 2014, 16,
963–4968.
1
1
1
1
6
4
2 G. Ren, M. Wang, L. Wang, Z. Wang, Q. Chen, Z. Xu and
D. Sun, Langmuir, 2018, 34, 5798–5806.
3 S. Fujii, Y. Cai, J. V. M. Weaver and S. P. Armes, J. Am. Chem.
Soc., 2005, 127, 7304–7305.
42430 | RSC Adv., 2020, 10, 42423–42431
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
14 M. Motornov, R. Sheparovych, R. Lupitskyy, E. Macwilliams, 30 C. Y. Xie, S. X. Meng, L. H. Xue, R. X. Bai, X. Yang, Y. Wang,
O. Hoy, I. Luzinov and S. Minko, Adv. Funct. Mater., 2007, 17,
307–2314.
Z. P. Qiu, B. P. Binks, T. Guo and T. Meng, Langmuir, 2017,
33, 14139–14148.
2
1
1
5 J. Li and H. D. H. Stover, Langmuir, 2008, 24, 13237–13240.
6 M. F. Haase, D. O. Grigoriev, H. Moehwald, B. Tiersch and
D. G. Shchukin, J. Phys. Chem. C, 2010, 114, 17304–17310.
7 J. Kim, L. J. Cote, F. Kim, W. Yuan, K. R. Shull and J. Huang,
J. Am. Chem. Soc., 2010, 132, 8180–8186.
31 J. Jiang, Y. Ma, Z. Cui and B. P. Binks, Langmuir, 2016, 32,
8668–8675.
32 G. Ren, X. Zheng, H. Gu, W. Di, Z. Wang, Y. Guo, Z. Xu and
D. Sun, Langmuir, 2019, 35, 13663–13670.
33 J. Tang, R. Berry and K. C. Tam, Biomacromolecules, 2016, 17,
1748–1756.
1
1
8 H. Liu, C. Wang, S. Zou, Z. Wei and Z. Tong, Langmuir, 2012,
2
8, 11017–11024.
34 Y. Chen, Y. Bai, S. Chen, J. Ju, Y. Li, T. Wang and Q. Wang,
ACS Appl. Mater. Interfaces, 2014, 6, 13334–13338.
1
2
9 F. Tu and D. Lee, J. Am. Chem. Soc., 2014, 136, 9999–10006.
0 X. Pei, K. Zhai, C. Wang, Y. Deng, Y. Tan, B. Zhang, Y. Bai, 35 B. Brugger and W. Richtering, Adv. Mater., 2007, 19, 2973–
K. Xu and P. Wang, Langmuir, 2019, 35, 7222–7230. 2978.
1 Y. Zhu, T. Fu, K. Liu, Q. Lin, X. Pei, J. Jiang, Z. Cui and 36 J. Tang, M. F. X. Lee, W. Zhang, B. Zhao, R. Berry and
B. P. Binks, Langmuir, 2017, 33, 5724–5733. K. C. Tam, Biomacromolecules, 2014, 15, 3052–3060.
2 X. Wang, M. Zeng, Y. H. Yu, H. Wang, M. S. Mannan and 37 H. Yang, T. Zhou and W. Zhang, Angew. Chem., 2013, 52,
Z. Cheng, ACS Appl. Mater. Interfaces, 2017, 9, 7852–7858. 7455–7459.
3 J. O. Zoppe, R. A. Venditti and O. J. Rojas, J. Colloid Interface 38 M. Pera-Titus, L. Leclercq, J. M. Clacens, F. De Campo and
Sci., 2012, 369, 202–209. V. Nardello-Rataj, Angew. Chem., 2015, 54, 2006–2021.
4 T. Saigal, H. Dong, K. Matyjaszewski and R. D. Tilton, 39 S. Wiese, A. C. Spiess and W. Richtering, Angew. Chem., 2013,
Langmuir, 2010, 26, 15200–15209. 52, 576–579.
5 B. P. Binks, R. Murakami, S. P. Armes and S. Fujii, Angew. 40 Z. Fang, D. Yang, Y. Gao and H. Li, Colloid Polym. Sci., 2015,
Chem., 2005, 44, 4795–4798. 293, 1505–1513.
6 Q. Jiang, N. Sun, Q. Li, W. Si, J. Li, A. Li, Z. Gao, W. Wang and 41 J. Huang and H. Yang, Chem. Commun., 2015, 51, 7333–7336.
2
2
2
2
2
2
2
2
2
J. Wang, Langmuir, 2019, 35, 5848–5854.
42 S. Yu, D. Zhang, J. Jiang, Z. Cui, W. Xia, B. P. Binks and
H. Yang, Green Chem., 2019, 21, 4062–4068.
7 Y. Chen, Z. Li, H. Wang, Y. Pei, Y. Shi and J. Wang, Langmuir,
2018, 34, 2784–2790.
43 B. Yao, Q. Fu, A. Li, X. Zhang, Y. Li and Y. Dong, Green Chem.,
2019, 21, 1625–1634.
8 R. Bai, L. Xue, R. Dou, S. Meng, C. Xie, Q. Zhang, T. Guo and
T. Meng, Langmuir, 2016, 32, 9254–9264.
9 Z. Chen, L. Zhou, W. Bing, Z. Zhang, Z. Li, J. Ren and X. Qu, J.
Am. Chem. Soc., 2014, 136, 7498–7504.
44 J. Tang, X. Zhou, S. Cao, L. Zhu, L. Xi and J. Wang, ACS Appl.
Mater. Interfaces, 2019, 11, 16156–16163.
45 M. Benaglia, E. Rizzardo, A. Alberti and M. Guerra,
Macromolecules, 2005, 38, 3129–3140.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 42423–42431 | 42431