Facile construction of thermo-responsive Pickering emulsion for esterification reaction in phase transfer catalysis system

Article abstract of DOI:10.1016/j.mcat.2020.111335

In this paper, a thermo-responsive Pickering emulsion (PE) is constructed for the esterification reaction in the phase transfer catalysis (PTC) system. Silica nanoparticle modified by alkyl polyoxyethylene ether and tetrabutylammonium bromide (TBAB) is the highly efficient thermo-responsive Pickering emulsifier to stabilize the diisopropyl ketone-in-water emulsion. PEs can keep stable at room temperature for 3 days and demulsify only in the condition of elevated temperature and agitation, which causes the easy product separation and aqueous phase recycling. The adsorption of TBA⁺ and nonionic surfactant via electrostatic interaction and hydrogen bonding, respectively, can increase the hydrophobicity and the emulsifying capacity of silica nanoparticle. The ion-pair of TBA⁺ and reactant anion is generated and can be transferred into the organic phase, initiating the bond-forming reaction. The modifying effect of polyoxyethylene ether is weakened with the increase of temperature due to the loss of the hydrogen bonding interaction, resulting in the demulsification of PEs. Moreover, the cloud point of polyoxyethylene ether is the temperature for the complete separation of two phases. In this PTC system, the conversion rate for the esterification can reach to 92 % and the aqueous phase can be reused at least 5 times without sacrifice of the catalytic activity, demonstrating the potential application in industry.

Full text of DOI:10.1016/j.mcat.2020.111335

Molecular Catalysis 500 (2021) 111335
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Facile construction of thermo-responsive Pickering emulsion for
esterification reaction in phase transfer catalysis system
a
,
b
a
,
b
a,
b
a
,
b
a,
b
a,b
Lihui Yang , Xiao Zhao , Manjun Lei , Jie Sun , Lei Yang , Yifeng Shen
,
a,c,
Qiangqiang Zhao
*
a
Key Laboratory of Advance Textile Materials and Manufacturing Technology, Ministry of Education, College of Textile Science and Engineering (International Institute
of Silk), Zhejiang Sci-Tech University, Hangzhou, Zhejiang, 310018, China
b
Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, College of Textile Science and Engineering (International Institute of Silk),
Zhejiang Sci-Tech University, Hangzhou, Zhejiang, 310018, China
c
Key Laboratory of Science & Technology of Eco-Textile, Ministry of Education, College of Chemistry, Chemical Engineering and Biotechnology, Donghua University,
Shanghai, 201620, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this paper, a thermo-responsive Pickering emulsion (PE) is constructed for the esterification reaction in the
phase transfer catalysis (PTC) system. Silica nanoparticle modified by alkyl polyoxyethylene ether and tetra-
butylammonium bromide (TBAB) is the highly efficient thermo-responsive Pickering emulsifier to stabilize the
diisopropyl ketone-in-water emulsion. PEs can keep stable at room temperature for 3 days and demulsify only in
the condition of elevated temperature and agitation, which causes the easy product separation and aqueous
Pickering emulsion
Phase transfer catalysis
Thermo-responsive
Esterification reaction
SiO
2
+
phase recycling. The adsorption of TBA and nonionic surfactant via electrostatic interaction and hydrogen
bonding, respectively, can increase the hydrophobicity and the emulsifying capacity of silica nanoparticle. The
+
ion-pair of TBA and reactant anion is generated and can be transferred into the organic phase, initiating the
bond-forming reaction. The modifying effect of polyoxyethylene ether is weakened with the increase of tem-
perature due to the loss of the hydrogen bonding interaction, resulting in the demulsification of PEs. Moreover,
the cloud point of polyoxyethylene ether is the temperature for the complete separation of two phases. In this
PTC system, the conversion rate for the esterification can reach to 92 % and the aqueous phase can be reused at
least 5 times without sacrifice of the catalytic activity, demonstrating the potential application in industry.
1
. Introduction
Conventional emulsions, which is stabilized by surfactants, cannot
Therefore, PEs avoid the need for the vigorous agitation to achieve high
conversion rates. PEs have been applied in biphasic catalysis system,
such as enzymatic catalysis, transition-metal nanoparticle catalysis and
phase transfer catalysis (PTC) [7–12], and exhibit high industrial
application potential.
be stored for a long stage due to its thermodynamically instability [1]
and reversible adsorption of surfactant at the interface. The emulsion
stabilized by the solid particles is a Pickering emulsion (PE) [2], which
differs from the conventional emulsion, on account of the irreversible
adsorption of solid particles [3,4]. Therefore, PEs are extremely stable
both thermodynamically and kinetically. It is well-know that PEs have
become an innovative platform for high-efficiency biphase catalytic
system due to its large interface area, high stability and without the need
for agitation [5]. The droplets in PEs act like a micro reactor. The
compartmentalized microspaces provide large reaction interface and
promote the mass transfer of reactants in the catalytic reaction [6].
Ester compounds have a wide range of application in the food,
pharmaceutical, textile and cosmetic industries [13]. The reaction with
organic halides and carboxylates is one of the most commonly used
methods for the ester compounds synthesis. The novel catalytic systems
such as enzymatic catalysis [14,15], ionic liquids [16,17], electro-
catalysis [18,19] and solid acid catalysis [20,21] have been adopted for
achieving high conversion rates. However, these reaction systems are
always carried out with the high temperature and stirring speed.
Therefore, high energy consumption is a great hindrance in the
*
Corresponding author at: Key Laboratory of Advance Textile Materials and Manufacturing Technology, Ministry of Education, College of Textile Science and
Engineering (International Institute of Silk), Zhejiang Sci-Tech University, Hangzhou 310018, Zhejiang, China.
E-mail address: qqzhao@dhu.edu.cn (Q. Zhao).
https://doi.org/10.1016/j.mcat.2020.111335
Received 5 September 2020; Received in revised form 1 December 2020; Accepted 2 December 2020
Available online 26 December 2020
2
468-8231/© 2020 Elsevier B.V. All rights reserved.

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
industrial production for these reaction systems. The introduction of
PTC technology in the synthesis of ester compounds, which brings dra-
matic improvement of the product yield, the reduction of cost and
environmental friendliness [22], has received researchers’ attention in
recent years. However, for the conventional PTC system, the agitation is
required to generate enough interfacial region and transfer the anion
reactant. It is urged to propose a new PTC reaction system that can
achieve the high conversion rate under low energy consumption
conditions.
(CTAB) (99 %), p-nitrobenzyl bromide (99 %), o-nitrobenzyl bromide
(99 %), biphenyl (99 %) and diisopropyl ketone (98 %) were purchased
from Aladdin Chemical Reagent Company (Shanghai, China). Fatty
alcohol polyoxyethylene ether 7 (AEO-7, >98 %) and fatty alcohol
polyoxyethylene ether 9 (AEO-9, >98 %) were provided by Shandong
Yousuo Chemical Co., China.
2.2. Preparation of emulsions
We have constructed common PTC reactions in the PE system. In PEs,
PTC reaction was carried out without stirring and the high conversion
rate was found. However, in practical applications, stable emulsion is
crucial in a specific period and has to be demulsified in the production
separation and the recycling process [23]. The high resistance of the PE
droplets aggregation increases the difficulty of demulsification. The
centrifugation is the common demulsification method for PEs. However,
centrifugation processes always cause low efficiency, high-energy con-
sumption and incomplete recovery. Stimulus-responsive PEs that can
activate and deactivate in response to the external trigger provide easy
separation and good recyclability of two phases. So far, the reported
stimulus-responsive PEs include thermo-responsive system [24,25],
salt-responsive system [26], electric or magnetic field-responsive system
Sodium acetate (0.02 mol) and water (7 mL) were placed into a 20
mL glass bottle (Φ27*57 mm). Then, the silica nanoparticle (1 wt.%) and
cationic surfactants (3 %) or nonionic surfactants (3 mM) or both were
added in order. Subsequently, diisopropyl ketone (7 mL) was added. The
mixture was operated at 12,000 rpm for 2 min by using an Ultraturrax
T25 homogenizer. Surfactants include nonionic (AEO-n) or cationic
surfactants or both, wherein the concentrations of the nonionic and
cationic surfactants are expressed in terms of the molars per liter relative
to the water phase and the molar ratio of the p-nitrobenzyl bromide,
respectively.
2.3. Catalytic reactions
Sodium acetate (2.85 mol L 1), silica particle (1 wt.%) and the
ꢀ
[
[
27,28], pH-responsive system [29–32] and gas-responsive system
33–35]. Although the gas-responsive system is environmentally benign,
modifiers (3 % of TBAB, 3 mM of AEO-7) were added to a glass bottle in
◦
hash conditions are always involved, such as high temperature or vac-
uum [36]. The pH-responsive system is easy to operate, but the addition
of acids or bases is required, which affects the chemical composition of
the systems [37]. The thermo-responsive system, which avoids the
changes of the chemical makeup in the system and is convenient in
operation [37], has been one of the most practical stimulus-responsive
PEs. However, as far as we know, few attentions are paid to construct
PTC reaction in thermo-responsive PE system.
sequence and allowed to stand at 30 C overnight. Then the diisopropyl
ketone solution of p-nitrobenzyl bromide (0.0015 mol) and diphenyl
(0.0005 mol) was added. PE was formed using an TURRAX T18 ho-
mogenizer at 12,000 rpm for 2 min. Furthermore, PE was maintained in
◦
◦
water at 57 C. After the complete reaction, PE was stirred at 80 C with
150 rpm. Subsequently, the oil phase and the aqueous phase were
separated by decantation. The conversion rate was determined by HPLC.
The aqueous phase was added into the new reactor with the fresh oil
phase containing p-nitrobenzyl bromide (0.0015 mol) and diphenyl
(0.0005 mol). After the homogenization of the two phases, the next
catalytic cycle was initiated.
The reported thermo-triggered colloid particles for PEs can be clas-
sified into two kinds: polymeric particles [38–40] and inorganic parti-
cles [41,42]. Commonly, the temperature-triggered functional group is
grafted on the surface of the polymer and inorganic particles, which
involves the complicated synthesis processes and limits the application
in large scale [43,44]. Cui’s group [45] reported a simple protocol to
prepare thermo-responsive PEs with hydrophilic silica nanoparticles in
combination with alkyl polyoxyethylene ether nonionic surfactant as
emulsifier. This PE is stable at room temperature and demulsify at
elevated temperature. Meanwhile, according to our previous research,
the cation catalyst in PTC system can also modify the hydrophilic silica
nanoparticles. Therefore, this simple and facile thermo-responsive PE
demonstrates the application potential for PTC reaction.
The organic phase was washed with HCl aqueous solution (pH 5),
dried in anhydrous magnesium sulphate, filtered and then evaporated in
vacuo. The residue was recrystallized by petroleum ether. Then, the
◦
obtained product was dried in vacuum at 45 C for 24 h.
2.4. Characterization
The emulsion type was confirmed using the drop test. After the ho-
mogenization, a drop of emulsion was added to the aqueous or oil
phases, respectively. When the emulsion was dispersed in the aqueous
phase but not diffused in the oil phase, water was the continuous phase
and O/W type emulsion was formed. If the emulsion was dispersed in the
oil phase but not diffused in the aqueous phase, W/O type emulsion was
formed.
Herein, thermo-responsive PEs based on silica particles modified by
non-ionic surfactant were constructed for the synthesis of benzyl esters
derivates with PTC. Effects of the quaternary ammonium cation and
nonionic surfactant on the emulsion stability and catalytic efficiency
were investigated. Subsequently, the modified process for silica and
demulsification mechanism for the thermo-responsive PEs were
analyzed. Finally, the synthesis of various benzyl esters derivates was
carried out in this thermo-responsive PEs and the reusability of the
aqueous phase was studied.
Microscopic pictures of emulsion droplets were obtained using an
optical microscope (Lv100 N POL; Nikon). Emulsion was diluted with
the continuous phase on a glass slide. Obtained images were analyzed
using Nano Measurer software and the average size of the emulsion
droplets was obtained.
Zeta potential of silica particle in water containing TBAB/AEO-7/
sodium acetate was measured using a MALVERN Zetasizer Nano ZS in-
strument. The concentrations of silica particle and sodium acetate were
maintained at 1 wt.% and 2.85 M respectively.
2
. Experimental
2
.1. Chemical
The contact angles were performed on an optical drop shape
analyzer. The aqueous phase was allowed to stand overnight to achieve
the distributional equilibrium. Then, the modified silica and the aqueous
phase were separated by centrifugation. The modified silica was dried in
Silica nanoparticles (primary diameter 20 nm, 30 wt.% silica and 70
wt.% water, pH 7.1) were provided by Zhejiang Yuda Chemical Co. Ltd.
Sodium formate (99.5 %), sodium acetate (99 %), sodium propionate
◦
(
(
99 %), sodium butyrate (99 %), tetrabutylammonium bromide (TBAB)
99 %), tetraethylammonium bromide (TEAB) (99 %), benzyl-
a vacuum oven at 60 C for 24 h. Subsequently, the powder was pressed
into the disk. The disk was placed into the oil phase and a drop of
triethylammonium bromide (TEBA) (98 %), dodecyl trimethylammo-
nium bromide (DTAB) (99 %), cetyltrimethylammonium bromide
aqueous phase (2
μ
L) was released. The images of the drop on the
modified silica disk under oil phase were recorded, and the contact
2

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 1. Photographs of diisopropyl ketone-in-water emulsions stabilized by (a) mixtures of silica nanoparticles (1.0 wt.%) and 3 mM AEO-7, (b) mixtures of 1.0 wt.%
silica nanoparticles and 3 mM AEO-7, (c) mixtures of 1.0 wt.% silica nanoparticles, 3 mM AEO-7 and 3 %TBAB, (d) the same as (c), (e) mixtures of 1.0 wt.% silica
nanoparticles and 3 %TBAB, (f) mixtures of 3 mM AEO-7 and 3 %TBAB, (g, h) Photographs of demulsification of the emulsion stabilized by 1.0 wt.% silica
◦
nanoparticles and 3 % TBAB and 3 mM AEO-7 at 75 C without agitation and with agitation respectively. (i, j) Optical micrograph and diameter distribution of (c).
The aqueous phase of b-h contains 2.85 M sodium acetate solution. Temperature = 25 ◦C. Scale bar represents 200
μ
m.
angles were measured using the software.
3. Results and discussion
The concentration of benzyl bromide in the organic phase was
determined by HPLC analysis with internal standard method. A drop of
reaction mixture was withdrawn at regular intervals and diluted to 5 mL
with solution consisting acetonitrile-water (60:40). Zorbax Eclipse XDB-
3.1. Emulsions stabilized by silica particles modified with surfactants and
quaternary ammonium cation
C18 column (150 mm x 0.46 mm, 0.5
μ
m) was chosen with acetonitrile-
According to the previous research, the PE, which was formed with
silica nanoparticles and nonionic surfactant, could remain stable for a
long time [46], as shown in Fig. 1a. However, in the PTC system, the
aqueous phase contains relatively high concentration of salt, which
remarkably influences the emulsifying capacity of the modified silica
particle. The stable emulsion is not formed when the aqueous phase
ꢀ
1
water (60:40) as the mobile phase. The flow rate was 1.0 mL min and
the detection wavelength was set at 270 nm.
NMR spectra were recorded on a Bruker DPX 400 spectrometer at
1
13
4
3
00 MHz for H NMR, 100 MHz for C NMR in CDCl .
Fig. 2. Photographs of diisopropyl ketone-in-water emulsions stabilized by mixtures of 1.0 wt.% silica nanoparticles and 3 mM AEO-7 and different cationic sur-
factant (TBAB, TEAB, TEBA, DTAB, CTAB) taken at 5 h after preparation; The aqueous phase contains 2.85 M sodium acetate solution. Temperature = 25 ◦C.
3

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 3. (a, b) Photographs of diisopropyl ketone-in-water emulsions stabilized by mixtures of 1.0 wt.% silica nanoparticles and 3 mM AEO-7 and TBAB of different
concentrations; from left to right: 2 %, 3 %, 4 %, 6.66 % and 10 %, taken 0.5 h, 5 h after preparation respectively. The aqueous phase of all systems contains 2.85 M
sodium acetate solution. Temperature =57 ◦C.
contains 2.85 M sodium acetate (Fig. 1b).
in combination with 6.43 mM of TBAB and 6.43 mM of TBAB in com-
bination with AEO-7 (3 mM) were carried out (Fig. 1e and f). The
emulsions were not formed. It indicates that silica particles modified by
TBAB and AEO-7 can emulsify the reaction mixture and silica nano-
particles play the crucial role in the generation of the emulsion.
Therefore, the obtained emulsion is the PE. The responsiveness of the PE
However, it is observed that stable and gel-like emulsion is generated
with the addition of TBAB (Fig. 1c). And even the emulsion is turned
upside down, the emulsion does not flow down (Fig. 1d). Meanwhile, the
average droplet diameter is 155
μ
m as shown in Fig. 1i and j. Subse-
quently, control experiments only containing silica particles (1.0 wt.%)
Fig. 4. (a-e) Optical micrographs of gel-like emulsion stabilized by a mixture of silica nanoparticles (1.0 wt.%) and 3 mM AEO-7, at selected TBAB concentrations of
2
, 3, 4, 6.66 and 10 % and (f) average droplet diameter taken 0.5 h after preparation. Temperature =57 ◦C. Scale bars represent 200
μ
m.
4

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 5. (a, b) Photographs of diisopropyl ketone-in-water emulsions stabilized by mixtures of 1.0 wt.% silica nanoparticles and 3 % TBAB, at selected AEO-7
concentrations of 0.1, 0.5, 1, 3 and 5 mM taken 0.5 h, 5 h after preparation respectively. The aqueous phase of all systems contains 2.85 M sodium acetate solu-
tion. Temperature =57 ◦C.
stability to temperature is also observed. The stable and gel-like emul-
3.3. Effect of the concentrations of TBAB on stability of PEs
◦
sion can be demulsified quickly in a water bar at 75 C by gentle
agitation (ca. 100 rpm) (Fig. 1h). Similar to conventional PEs with AEO-
Visual observation of the emulsions with the concentrations of TBAB
ranged from 2 % to 10 % are shown in Fig. 3. The stability of the
emulsions is increased with the increasing concentration of TBAB. Stable
emulsions cannot be formed until the initial concentration of TBAB in
aqueous phase is higher than 6.43 mM (3 %). Meanwhile, the aqueous
phase is always precipitated in the bottom layer of the emulsions in the
bottle.
7
7
, no coalescence is observed in the upper layer after standing for 5 h at
◦
5
C without stirring (Fig. 1g), which is attributed to the fact that
stirring can accelerate the contact chance of the droplet.
3
.2. Effect of quaternary ammonium salt on stability of PEs
To examine the effect of cations on the formation of PEs, tetraethy-
The size and distribution of droplets are the key factors that affect the
stability of the emulsions. The photomicrographs of the emulsion and
the average droplet size map and diameter distribution are shown in
Fig. 4. The droplet size of these gel-like emulsions decreases from 177.8
lammonium bromide (TEAB), benzyltriethylammonium bromide
TEBA), dodecyl trimethylammonium bromide (DTAB) and cetyl-
(
trimethylammonium bromide (CTAB) are used as the modifiers
respectively. It can be seen that the emulsions containing TEAB or TEBA
can keep stable. However, the emulsions flow down quickly after
turning upside down (Fig. 2). The emulsions cannot be stabilized with
DTAB and CTAB as the modifiers (Fig. 2).
μ
m to 84.5 m (Fig. 4f). No significant change in average droplet size is
μ
observed with more than 6.43 mM (3 %) of TBAB used, as shown in
Fig. 4f. It indicates that the increase in the concentration of TBAB has no
influence on the stability of the emulsion and the droplet diameter when
the concentration of TBAB in aqueous phase is up to 6.43 mM (3 %).
These results of the average droplet diameters are consistent with
that for the stability of the emulsions. The adsorption of TBAB on the
particle surface improves the hydrophobicity of silica particle. As the
The hydrophilic-hydrophobic balance and volume of the cations are
the main factors to influence the modification to silica nanoparticles.
DTAB and CTAB have a higher hydrophobicity than TBAB, TEAB and
ꢀ
-
+
+
TEBA. The ion pairs of Ac with DTA or CTA are generated and prone
to dissolve in the organic phase. In this case, the ion pairs will reduce the
concentration of DTAB or CTAB in aqueous phase and foster the
desorption of DTAB or CTAB from silica particle surface [47]. Therefore,
+
initial concentration of TBAB increases, the amount of TBA adsorbed
on the particle surface increases, and the hydrophobicity of the modified
particles is enhanced. Therefore, the emulsifying ability of the particles
is enhanced, and the stable PE can be formed.
2
the lipophilicity of the modified SiO is lower than that for TBAB, TEAB
and TEBA and the modified silica exhibits the relatively low surface
activity. TEAB and TEBA are more hydrophilic than TBAB and the vol-
umes are less than TBAB. The modified silica for TEAB or TEBA is tighter
than that for TBAB. Therefore, the cross network between the modified
In the previous research [47,48], it is found that the second layer for
the modifier on the surface of silica exists, and the hydrophobicity of the
modified silica decreases. As a result, the stability of the emulsion will
decrease to some extent. However, this phenomenon is not found in this
PE. The stability of the emulsion corresponding to 21.4 mM (10 % to
mole of the p-nitrobenzyl bromide) of TBAB is still better than that
corresponding to 6.43 mM of TBAB. Therefore, the second layer
adsorption cannot be generation, which is also certified in the following
SiO
2
is hardly generated. The viscosity of the emulsion is much less than
that for TBAB inducing a high fluidity.
5

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 6. (a-e) Optical micrographs of gel-like emulsion stabilized by a mixture of silica nanoparticles (1.0 wt.%) and 3 % TBAB, at selected AEO-7 concentration at 0.1,
0
.5, 1, 3 and 5 mM and (f) average droplet diameter taken 0.5 h after preparation. Temperature =57 ◦C. Scale bars represent 200
μ
m.
Fig. 7. Reaction conversion of an emulsion containing (a) 3 mM AEO-7 with different TBAB concentrations. (b) 3 % TBAB with different concentrations of AEO-7.
◦
Reaction conditions: 0.22 M of p-nitrobenzyl bromide, 2.85 M of sodium acetate, the reaction temperature is 57 C. Emulsion system: 7 mL/7 mL O/W Pickering
emulsion containing 1.0 wt.% silica particles.
experiment.
of TBAB as the modifier alone, and the surface activity of the modified
particle is not enough to stabilize the diisopropyl ketone-in-water
emulsion. Nonionic surfactant molecule can be adsorbed on the sur-
face of silica particle involving the hydrophobic headgroup exposing
toward organic phase, thereby improving the lipophilicity of the
modified particles. Meanwhile, the adsorbed amount on the surface of
silica particles increases with the usage of AEO-7 increasing, Then, the
lipophilicity of silica particles is further enhanced and a stable emulsion
with relatively smaller droplet is formed.
3
.4. Effect of the concentrations of AEO-7 on the stability of emulsions
Fig. 5 shows that the effect of concentrations of nonionic surfactant
on the stability of emulsions. It can be seen that the stability of gel-like
emulsion is enhanced upon increasing the usage of AEO-7. After
standing for 0.5 h, the height of the separated water phase decreases
with the increase in the usage of AEO-7. Meanwhile, the separated
organic phase is formed with 0.1 and 0.5 mM AEO-7 after standing for 5
h. However, no coalesced oil phase is observed when initial concentra-
tion of AEO-7 is more than 1 mM. The micrographs and mean size of the
emulsion droplets are shown in Fig. 6. The droplet size of the emulsions
with less than 1 mM AEO-7 tends to be less regular than that with 1 mM
AEO-7. The average diameters of the droplets gradually decrease when
the initial concentration of AEO-7 is varied from 1 mM to 5 mM. As
shown in Table S2, the Kurtosis values keep invariable with the increase
in the usage of AEO-7. However, the absolute value of the Skewness is
the smallest when the concentration of AEO-7 is 3 mM, indicating that
the droplet size is closest to the standard normal distribution.
3.5. Effect of the modifiers concentrations on conversion rate of the
reaction
To further explore the role of the modifiers in the whole catalytic
process, effects of the concentrations of cation (TBAB) and nonionic
surfactant (AEO-7) on the catalytic activity were studied (Fig. 7). The
conversion-time curves with various modifiers were shown in Figure S2
and Table S3 and S4. It can be seen that these curves can be fit by the
pseudo-first order kinetic model well and the pseudo-first order reaction
constants can be got.
The modification of the particle surface is insufficient with 6.43 mM
In this PTC system, the initial usage of TBAB is an important factor
6

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 8. Zeta potential of (a) 1.0 wt.% silica nanoparticles dispersed in different concentrations of TBAB in aqueous solutions, (b) a mixed solution of 1.0 wt.% silica
and 3 % TBAB as a function of the concentration of AEO-7, which contains a sodium acetate solution. (c) The adsorption process of modifier (TBAB and AEO-7) on the
surface of a silica particle.
affecting the reaction rate. The reaction rate constants increase from
3.6. Mechanism of stabilization/destabilization of emulsions
ꢀ 1
ꢀ 1
0
.937 h to 3.161 h upon the initial usage of TBAB increasing from 2
to 10 %. The linear relationship between the usage of catalyst and the
%
To explore the stabilization/destabilization mechanism of PE, the
effect of concentrations of TBAB and AEO-7 on the zeta potential of silica
particles was investigated. As shown in Fig. 8, the zeta potential of the
silica nanoparticles is -37.7 mV in pure water. The negative charge of the
particles tends to be progressively neutralized upon increasing the
concentration of TBAB, thus leading to the decreasing magnitude of the
zeta potential (negative). The zeta potential value no longer increases
when the concentration of TBAB is up to 6.43 mM (3 % to mole of the p-
nitrobenzyl bromide). In the dispersion system containing sodium ace-
tate solution, the zeta potential of colloid is -4.09 mV without TBAB
(Fig. 8a). With the increase in the usage of TBAB, the zeta potential
progressively increases, which is higher than that of the modified par-
ticles without sodium acetate. The zeta potential value no longer in-
creases when the concentration of TBAB is up to 5 %. Without AEO-7 in
the aqueous phase, the zeta potential of the particle is ꢀ 3 mV as shown
in Fig. 8b. As the addition of AEO-7, the magnitude of the zeta potential
gradually decreases until the concentration of AEO-7 up to 1 mM, after
which there is a sudden reversal of the sign [50] and magnitude of the
zeta potential becomes positive.
reaction rate constants was observed, which is similar to that in the
conventional PTC systems. However, the increase in TBAB usage has
limited effects on the diameter of emulsion droplets. Effect of AEO-7
usage on the reaction rate is shown in Figure S2d and Table S4. The
ꢀ 1
ꢀ 1
reaction rate constants increase from 0.363 h to 0.950 h upon initial
concentration of AEO-7 increasing from 0.1 mM to 5 mM.
The effect of concentrations of surfactants on the reaction rate con-
stant indicates that the two modifiers have different effects for the cat-
alytic cycle. The cation (TBAB) performs as phase transfer catalyst, and
nonionic surfactant (AEO-7) corresponds to the stabilizer of emulsion.
+
Ion pairs between TBA and the acetate anion are formed in the aqueous
phase, which can be transferred into the interfacial region and react with
the p-nitrobenzyl bromide [49]. The higher the concentration of added
TBAB is, the greater the number of ion pairs formed in the system is,
leading to the increase in catalytic efficiency. For non-ionic surfactant,
the diameter of emulsion droplet decreases with the increase in the
amount. Therefore, the area of the interfacial region is increased and the
mass transfer rate is enhanced.
When the cation is adsorbed on the surface of the particles, the silica
Fig. 9. (a, b) Evolution of contact angles for water droplet on the modified silica in diisopropyl ketone with different concentration of TBAB and AEO-7, respectively.
7

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Fig. 10. (a) Fraction of emulsion as oil phase,
f
o
, versus temperature for emulsion stabilized
by silica nanoparticles (1.0 wt.%) and 3 % TBAB
and 3 mM AEO-7 or AEO-9 at Ф = 0.5. (b)
w
Time required to achieve complete phase sepa-
ration of gel-like emulsion stabilized by silica
nanoparticles (1.0 wt.%) and 3 % TBAB and 3
mM AEO-7 at various temperature with gentle
agitation (100 rpm). (c) Schematic representa-
tion of possible mechanisms for emulsification/
demulsification transition of silica particles and
surfactant stabilized Pickering emulsion.
◦
◦
particles become surface activity. However, only monolayer adsorption
of TBAB on the surface of colloid particles is formed. The second
adsorption cannot be achieved due to its large hydrophobic groups. In
other words, the hydrophobic groups of TBAB take up more space than
that of CTAB, which renders the amount of TBAB adsorbed on the silica
particles is limited. Therefore, the zeta potential of silica particles no
longer increases after the adsorption saturation.
concentration, from 81 (3 mM) to 74 (5 mM) (Fig. 9b).
The increase in contact angle with the usage of modifiers shows the
increase in the surface-active of the modified silica particle. Hence, the
results are consistent with the conclusion that emulsion only can be
stabilized by particles with suitable hydrophobicity. However, the
loosen structure of TBAB also induces the decrease in contact angle.
With the presence of high-concentration salts, the sodium acetate
dissolved in water can also stick to the surface of silica particles, which
renders the loss of charge of colloid particles and the increase of zeta
potential compared with that of salt-free system. Meanwhile, the ion
3
.7. Thermo-responsive character of PEs
The stability of the emulsions containing AEO-7 or AEO-9 was
+
characterized, as shown in Fig. 10. The degree of demulsification is
related to the demulsification temperature. Fig. 10a shows that the ratio
pairs between TBA and acetate anion is also formed. Therefore, the
+
competitive adsorption of TBA between SiO
2
and acetate anion exists,
of separated oil phase (f
o
◦
) increases upon increasing temperature below
and more cations are required to achieve saturation adsorption on the
surface of silica particles. It is similar to the studies that the presence of
high-concentration salts causes an increase in the zeta potential of the
dispersion system [51–53].
With dissolved in water, AEO-7 can also be adsorbed on the surface
of silica particle via the hydrogen bond between the hydrophilic group
and the particle. The hydrophobic chain of AEO-7 is a longer single
chain, rendering the higher lipophilicity of silica particle modified by
nonionic surfactant alone than by cation (TBAB) alone. It is believed that
the adsorption of AEO-7 on particles is in the form of hemimicelle when
the usage of AEO-7 is beyond 3 mM, rendering the fact that the hydro-
philic groups are exposed towards water.
◦
5
7 C for AEO-7 and 75 C for AEO-9. When the temperature rises to 57
◦
◦
C and 75 C, the emulsions containing AEO-7 and AEO-9 are completely
demulsified respectively.
As the temperature increases, the demulsification is significantly
accelerated, and the time for the emulsion to achieve the complete phase
separation decreases (Fig. 10b). The linear relationship between the
time for the complete phase separation and temperature is observed,
◦
when the temperature is higher than 70 C [45].
The results show the oil phase is completely separated when tem-
perature is up to cloud point of nonionic surfactants. The nonionic
surfactant plays a vital role for the preparation of thermo-responsive
PEs. Nonionic surfactant molecules dissolve in the aqueous phase
through hydrogen bonding between the ether bonds and water mole-
cules. In this PTC system, nonionic surfactant is adsorbed onto nano-
particle surface via hydrogen bonding, which endows silica particles
with surface activity. The elevated temperature can increase the chance
of oil droplets colliding with each other and accelerate the diffusion of
alkyl polyoxyethylene ether, enhancing the desorption of the surfactant
molecule from the colloid surface and the further desorption of the silica
nanoparticles from the oil-water interface [54]. The water-soluble
nonionic surfactant has a cloud point. When the temperature is above
the cloud point, the hydrogen bond between particles and the nonionic
Then the effect of the concentration of two modifiers on the contact
angle of silica particles at the oil-water-quartz interface was investi-
gated. The contact angle for water drops under diisopropyl ketone
◦
progressively increases with the increasing TBAB usage, from 69 (0 %
◦
TBAB) to 81 (3 % TBAB) (Fig. 9a). Then, with the further increase in
◦
TBAB concentration, the contact angle decreases, from 81 (3 % TBAB)
◦
to 54 (10 % TBAB). The effect of AEO-7 usage on the contact angle is
similar to that of TBAB. The contact angle for water drops under diiso-
◦
propyl ketone progressively increases with the AEO-7 usage, from 57 (0
◦
mM) to 81 (3 mM) and decreases upon further increase in AEO-7
8

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
Table 1
3.8. Other esterification reactions
Esterification of alkali metal carboxylates in static PE system.
To demonstrate the versatility and efficiency of the prepared PTC
system, esterification reactions with various reactants were carried out.
It can be seen that the prepared PE system shows high reactivity
(Table 1). For all the substrates, the conversion of the esterified product
is over 90 % within 5 h. The longer the alkyl chain of the alkali metal
carboxylate is, the higher the reaction rate is.
Entry
Substrates
AEO-7 Conc.
mM)
T
Conversion
(%)
o
(
( C)
1
2
3
4
5
6
7
8
R
1
R
1
R
1
R
1
R
1
R
1
R
1
R
1
=H
3
3
5
5
3
3
5
5
65
57
57
57
65
57
57
57
96.13
98.89
99.59
100
=CH
=CH
=CH
=H
3
2
2
R
2
= 4-
CH
CH
3
NO
2
2
CH
3
3
94.71
91.64
97
In the PTC system, the ion pairs of the carboxylate with TBAB are
formed. With the increase of lipophilicity of the carboxylate, the ion
pairs are prone to enter the oil phase or at the water-oil interface.
Thereby the collision between the reactant molecules was accelerated,
leading to the increase in the catalytic efficiency. In addition, the reac-
=CH
=CH
=CH
3
2
2
R
2
= 2-
2
CH
CH
3
NO
2
CH
98.06
Reaction condition: 0.02 mol of carboxylates (In addition to 0.015 mol of sodium
butyrate), 1.5 mmol of benzyl bromide derivative; 3 % of TBAB; 1.0 wt.% SiO ; 7
2
tion of benzyl bromide derivatives with acid derivatives follows the SN
2
mL of diisopropyl ketone; 7 mL of water; All of the systems are emulsified with
an Ultra-turrax at 12,000 rpm for 2 min.
nucleophilic substitution mechanism. The inductive effect of electron-
donating groups in acid derivatives causes electron density of carbox-
ylic ions to increase and the inductive effect of electron-withdrawing
groups in benzyl bromide derivatives causes electron density of benzyl
group to decrease, which both can increase the reaction activity.
surfactant molecules is disrupted, which causes the surfactant to desorb
from the particle surface and promote its transfer to the oil phase
(
Fig. 10c). The silica colloid particles become hydrophilic again, even-
tually leading to demulsification [55].
Fig. 11. (a) Photographs of diisopropyl ketone-in-water emulsions following switching on and switching off cycles take 5 h after preparation. Add aqueous phase
containing sodium acetate, TBAB and AEO-7 to keep concentration of components consistent with the initial concentration after destabilization. (b) Reaction rate
constant of the emulsion (1st, 2nd, 3rd, 4th and 5th reuse). Reaction conditions: 0.22 M of p-nitrobenzyl bromide, 2.85 M of sodium acetate, the reaction temperature
◦
is 57 C. Emulsion system: 7 mL/7 mL O/W Pickering emulsion containing 1.0 wt.% silica particles.
9

L. Yang et al.
Molecular Catalysis 500 (2021) 111335
3
.9. Reuse of aqueous phase
Acknowledgements
The reusability of the PTC system was also investigated. When the
This work was financially supported by the Fundamental Research
Funds for the Central Universities (2232020G-04), National Natural
Science Foundation of China (21606206), and the Initial Research Funds
for Young Teachers of Donghua University.
new organic phase is added to the separated aqueous phase and reho-
mogenized, demulsification of the reaction mixture occurs rapidly
(
Figure S3b). However, once silica particle, AEO-7 and TBAB are added,
the stable emulsion is formed again (Figure S3a) and the PE also has
temperature responsiveness. The stability of emulsion depends only on
the presence and concentration of silica, TBAB and AEO-7 in the
aqueous phase. The distribution equilibrium of TBAB and AEO-7 be-
tween the organic phase and aqueous phase is established. The con-
centration of modifiers in aqueous phase is decreased and the stable
emulsion cannot be formed when only new organic phase is added.
Fig. 11 shows the photographs of the reused emulsion. As shown in
Fig. 11b and Figure S4, there is no significant difference on the reaction
conversion and the reaction rate constant when the aqueous phase is
reused 5 times. Therefore, thermos-responsive PE exhibits the excellent
reuse performance and the aqueous phase can be reused without sacri-
fice of the yield and reaction rate.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111335.
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1